Oh, I'm so sorry, you got very bad advice from the employee at Petco no doubt. That's awful.
I'm pretty much certain that he died of starvation/dehydration because you were feeding him the pellets in a bowl and had water in a bowl...That's not what Bearded Dragons eat, especially babies/juveniles that are under a year old, they must eat tons and tons of live insects every single day, on average babies/juveniles eat between 40-60 appropriately sized live insects every single day. They don't and won't eat those damn pellets, nor any other "commercial" food. They do most all of their growing and developing during their first year of life, and as a result they must be given at least 2, 3 is better, live insect feeding sessions every single day, each session lasting 10-15 minutes, where you give them AS MANY LIVE INSECTS AS THEY WANT. You have to do these live insect feeding sessions 3 times a day up until they are 8 months old, then scale it back to 2 sessions a day, then when they reach a year of so old they typically switch themselves over to eating more fresh greens and veggies with only 1 live insect feeding session a day. You also must dust all of the live insects from 1 of the sessions per day in a Calcium powder and do this 5 days a week, and in a Multivitamin powder 3 days a week, that's for the rest of their lives. You should also put a salad of FRESH greens and veggies that are appropriate for Dragons in their tank every single day...Once again, because they do most of their growing and developing during their first year of life, most Dragons who are under a year or so old will not eat much, if any greens or veggies, some don't eat any, some just nibble on them, and then there is a minority that eat a lot. Regardless you must give them a salad of FRESH, healthy greens and veggies every single day, after they have their first live insect feeding session of the day, so they can get used to seeing them, and are able to pick at them and get used to them, and like I said, at a year or so old they'll switch themselves over from 90% live insects and 10% fresh greens/veggies to almost the exact opposite...
Commercial Bearded Dragon "foods", like pellets, are not usually eaten by Dragons at all, especially not at all by babies or juveniles, because once again, they need tons and tons of live insects every single day. They wouldn't even know what those pellets were unless you force-fed them. I've used them before to make a slurry to feed to a sick dragon, but that's a totally different situation, as I'm talking about feeding a Dragon who has stopped eating on their own, and feeding them a liquid slurry of soaked pellets, baby food, pedialyte, and a pinch of calcium and multivitamin that is mixed in a blender, and feeding it to them with an oral syringe, and this is only a very short-term thing. You cannot have a Bearded Dragon and not feed them a healthy, adequate staple insect that is ALIVE. You cannot EVER feed them already dead or "freeze-dried" insects, as they obviously contain ZERO water, and since they get most all of their
hydration from live insects and fresh greens/veggies that they eat, they'll die of dehydration if you feed them pellets and/or freeze-dried insects or veggies. These commercial foods are a total waste of money, and again, this is why your Dragon's "stomach was caved in", he starved and dehydrated himself to death. Not your fault, please don't blame yourself, but rather learn from your mistakes so the next time you'll know exactly what to do...
Bearded Dragons for the most part DO NOT DRINK ANY WATER, AS THEY HAVE NO CONCEPT OF WHAT "STANDING WATER" IS, BECAUSE THERE IS VERY LITTLE TO NO STANDING WATER IN THE AUSTRALIAN DESERT WHERE THEY COME FROM NATURALLY. As a result, Dragons get most all of their
hydration from the live insects and the fresh greens and veggies that they eat daily. Some Dragons will lick up a little water from being misted, or they'll occasionally drink water from a
bath or a bowl, but that's the minority, most Dragons will never really drink water from a bowl or
bath, and they certainly cannot survive on the little bit of water they lick up from being "misted"; they are Desert Reptiles, not Tropical Reptiles, there's a huge difference. HUGE. When a Bearded Dragon stops eating/loses their appetite, for whatever reason, say they are sick or injured, or you have them under inappropriate lighting or temperature zones, then you must drip water from an oral syringe or eyedropper directly on their snouts and let them lick it off, drip, lick, drip, lick, etc. and let them have as much water as they'll lick off, and you must do this several times throughout the day, as they are getting no
hydration if they are not eating their live insects and fresh greens/veggies.
****Most Dragon owners, especially if the Dragon is under a year old, order their live staple insects online, in bulk. This is because #1)They eat so many of them per day, and #2) It is obviously not economical to buy that many live staple insects in a pet shop, as their prices are absolutely ridiculous. Also, shops like Petco and Petsmart do not have any variety of adequate staple insects for Bearded Dragons, in fact the only staple insect that they sell that is appropriate for a Dragon are Crickets. That's it. You cannot feed a Dragon mealworms as a staple insect because they cannot properly digest their shells, and they don't contain nearly enough protein, they are almost all fat and hard shell...THE ADEQUATE, LIVE STAPLE INSECTS FOR DRAGONS INCLUDE: Crickets, several species of Roaches, including Dubia Roaches, Silkworms, ButterWorms, and then Phoenix Worms/Black Soldier Fly Larva (BSFL)/ReptiWorms/CalciWorms/NutriGrubs (all the same thing, just different brand names)...Just as an example for you, Petco sells a container of 25 CalciWorms, most of which are tiny little small sized worms, for $4.25...A baby Dragon can eat 20 size large CalciWorms in one sitting, 3 times a day...I order mine from
www.dubiaroaches.com because they are the cheapest place I've found and they ship in 2 days, have auto ship, and they never ship me any dead worms...I order 1,000 size Large each month, only size Large CalciWorms (they call them BSFL) for $25 shipped!!! You see why you have to order in bulk online...
Now the other issue you have is that you had no
UVB light at all for your Dragon, again the Petco employee sold you a Basking Bulb that says it emits "UV Light", but that isn't
UVB light, it's UVA light, or just regular light...Being Desert Reptiles, Dragons need 13-14 hours every single day of strong, adequate
UVB light from a long (at least 18" or longer) UVB tube that is inside a long, fluorescent tube fixture that has a reflector inside it that sits behind the UVB tube; And their long UVB tube must be at least a 10% UVB Output tube, or at least a "10.0" UVB tube, a lower UVB output like a 5.0 tube is not adequate...There are several off brands of UVB tubes that must be avoided, including Zilla, All Living Things, ReptileOne, and Trixie, as they are weak and all emit harmful light rays that seem to always cause severe eye damage. You need to buy either a Reptisun 10.0 or an Arcadia brand UVB tube that is at the very least 18" long, 22" long or longer is better.
There are 2 strengths of long UVB tubes, the much weaker T8 and the much stronger T5. The main differences are: The weaker T8 tubes cannot sit on top of the mesh lid to the tank, they are too weak, so they must be strapped to the underside of the mesh lid using zip ties, wire, twine, etc., while the stronger T5 UVB tubes can sit on top of the mesh lid. #2) The T8 tubes must be within at least 6" of the Dragons Basking spot/Platform, while the stronger T5 UVB tubes must only be within about 10" of the Dragon's Basking spot/platform. #3) The T8 UVB tubes must be replaced once every 6 months on the dot, because they stop emitting any
UVB light at 6 months old, while the T5 tubes only need to be replaced once every 12 months. Obviously you want to buy a T5 UVB tube that is at least a 10% or 10.0 T5 tube and fixture.
Before you get another Dragon, you absolutely must buy a proper UVB tube...The most common reason that Dragons lose their appetites and become lethargic is because they are under inadequate UVB lights or the UVB lights are mounted incorrectly, too far away, obstructed, etc. or they are too old. WITHOUT 13-14 HOURS PER DAY, EVERY DAY, UNDER A STRONG, ADEQUATE UVB TUBE, BEARDED DRAGONS CANNOT ABSORB ANY OF THE CALCIUM THAT THEY EAT/INGEST IN THEIR FOOD OR IN THE SUPPLEMENT POWDER YOU GIVE THEM, AND THEY DEVELOP A CALCIUM DEFICIENCY AND METABOLIC BONE DISEASE (MBD) VERY QUICKLY. THIS IS THE MOST IMPORTANT PIECE OF EQUIPMENT YOU WILL BUY YOUR DRAGON.
Amazon.com is by far the cheapest place to order a long T5 UVB tube and fixture with a reflector for it. You can buy a 22" Reptisun T5HO UVB tube for about $25 shipped on Amazon.com. They also sell one particular fixture for it that is 24" long, is rated for a T5 tube (rated for 24 watts), and includes a reflector inside it for about $28 shipped. They do sell a couple of fixtures that are 24" fixtures that are T5-rated for a couple bucks less, but they do not include a reflector, and this is not optional. So before you get another Dragon, you must get a proper T5 UVB tube and fixture.
And also under a bright-white colored Basking Bulb (never use ANY colored bulbs for a Dragon, like red, blue, purple, black, etc.), that is the correct wattage to keep the Basking Spot/Platform Surface Temperature within the correct range, which for a Dragon under a year old is between 105-110 degrees F, and for a Dragon that is over a year or so old they tend to like their Basking Spot Surface Temperature a tiny bit lower, between 100-105 degrees F. This is the temperature that will determine whether or not they can properly digest the food that they eat, and is the most critical temperature zone inside their tank. That being said, YOU CANNOT MEASURE THEIR BASKING SPOT/PLATFORM SURFACE TEMPERATURE WITH ANY TYPE OF STICK-ON THERMOMETER!!! Stick-On Thermometers only measure Air/Ambient Temperature, not Surface Temps, so you absolutely must have either a Temperature Gun or a Digital Thermometer that has a Probe on a Wire that you can set directly on the Basking Spot/Platform. Then they also must have a Cool Side to the tank, the side opposite of the Hot Side, which is the side that has both of the lights overtop of it and contains the Basking Spot/Platform. The Cool Side Air/Ambient Temperature should be between 75-80 degrees F maximum, no higher, as they must always have a place to go and cool down.
You're trying to replicate natural sunlight over the Hot Side of the tank as closely as possible. You do this by putting the long, strong UVB tube over the Hot Side of the tank, and then putting the bright-white colored Basking Bulb right alongside it, so that both lights are "sharing" the top of the Hot Side of the tank. Then you put the Dragon's Basking Spot/Platform within the Hot Side of the tank, and directly underneath both lights, so your Dragon will get both lights at the same time while basking. BOTH LIGHTS MUST BE ON EVERY DAY FOR 13-14 HOURS. No nighttime heat source is needed at all as long as the tank Air/Ambient Temperature is at least 65 degrees. The Australian Desert is both PITCH-BLACK and COOL/COLD at nighttime, and as such Dragons must have a nighttime tank Air/Ambient Temperature that is considerably cooler than their daytime temperatures in order to sleep soundly and allow their bodies to rest and recover. Most people's homes are at least 63 degrees or above at nighttime, and as such most people don't need any type of nighttime heat source for their Dragons. A nighttime temperature between 65-72 degrees F is perfect.
If you had been feeding your Dragon live insects several times a day, then I'd be telling you that the issue, or main issue was a total lack of
UVB light, let alone strong, adequate
UVB light. However, even if you had had a long, very strong, T5 10.0 UVB tube over your Dragon, he still would not have been eating those pellets, at least not enough to sustain him, and he'd be getting little to no
hydration from being misted, and he probably did not drink much, if any water from the water bowl...And since he had a "caved-in" stomach and a
black beard, it's pretty clear that he died of starvation and dehydration...This isn't your fault, you were given horrible advice, really you were given no advice at all that was good...And this is unfortunately a very common issue, and it happens all the time. And not just at Petco, but at PetSmart, Petland, and tons of small, independent pet shops and even "reptile specialty" shops. The employees get no training or education at all about the different species of reptiles, birds, etc. that they sell, and with reptiles this just doesn't ever go well, especially with Desert Reptiles that have such specific diet, lighting, and temperature requirements...I'm so sorry, but now you'll know a lot more, and you can ask use ANY questions you have at any time, send PM's to any of us at any time, etc.
Did the 'heating lamp' emit light? Bearded dragons need a basking lamp that emits light and also a separate quality tube 
