First off, I'm so glad you found a certified reptile vet to take him to today, this is crucial. Please let us know what happens at the vet, as I'm hoping if the vet is an experienced reptile vet they will know exactly what needs to be done, and will educate you on how to do the tube feedings. As Kingofnobby mentioned he will most likely be kept in the hospital for at least a few days for feedings,
hydration, and medications (I'm going to assume that he has some infections and/or parasites/worms, as the prior owner certainly didn't keep his enclosure clean and was not concerned with hygiene at all. He most likely will not be eating on his own for quite a while due to a combination of being so weak and having no appetite, so even trying to feed him by dripping Critical Care or a similar nutritional supplement on his snout for him to lick off isn't going to work. "Tube feeding" literally means putting a special feeding "needle" (not sharp or for injections, just a long, hollow needle with a blunt end) that is attached to a syringe filled with the food down his esophagus and injecting the food right into his stomach. The key here is that the vet needs to show you how to be sure the feeding tube is in his esophagus/stomach and not his trachea, as that will put the food into his lungs instead of his stomach, resulting in major problems. Tube feeding is very, very easy once the vet shows you and you do it once, then it's not only easy but it's quick and you're certain to get the correct amount of food into his stomach, and it's actually less stressful for them after they do it a few times and know what to expect, rather than trying to pry his mouth open with an oral syringe and making him swallow. He will get all of the nutrition and fluids he needs through the tube feedings, and the vet may or may not also do subcutaneous fluid injections, which are also very easy to do once you're shown how.
Hopefully the vet will run a fecal test for parasites/worms and bacteria in his gastrointestinal tract, this is quite common. I don't know if the vet will be able to get a blood or fecal sample from him right away, so this is another reason they will keep him, so they can eventually get enough nutrition and fluids in him to be able to do testing for parasites and infections.
As far as you going out and buying things for his setup, we can definitely help you with what you need to get for him. CHOOSING THE CORRECT UVB TUBE LIGHT AND FIXTURE IS THE MOST IMPORTANT THING YOU NEED TO DO! Improper or no
UVB light is most likely what caused this to start in the first place, if a bearded dragon doesn't get 12-14 hours a day under an adequate
UVB light that is strong enough for a desert reptile such as they are, they will not only lose their appetite, but they will not be able to absorb any nutrition from any food or supplements that they do actually eat, including Calcium. This leads to both starvation and calcium deficiency. And unfortunately most of the UVB lighting out there that is marketed for bearded dragons is not adequate for them, so this is why this happens to a lot of people, they are given bad advice from pet shop employees (DO NOT LISTEN TO PET SHOP EMPLOYEES ABOUT LIGHTING!) about what lighting to buy.
I don't know what size enclosure you have for him to live in, or that you're going to buy for him, but the basics of his lighting are going to be the same, it's just a matter of the length of the UVB tube and fixture you buy and the wattage of the basking bulb that you buy in relationship to the size of the enclosure.
You need a long, flourescent UVB tube and a matching length flourescent tube fixture, not a coil or compact
UVB bulb that screws into a regular lightbulb fixture. Neither any brand of the coil or compact UVB bulbs emit a strong enough
UVB light for a bearded dragon, and coil bulbs also cause eye and skin damage. In the US, the Reptisun brand 10.0 (NOT 5.0) UVB tube is the best you can buy, usually you'll find it in either an 18" or 24" length. Most of the Reptisun 10.0 UVB tubes you'll find in pet stores will be T8 strength tubes, which is fine, but the fixture will need to be mounted INSIDE the enclosure using 3M Command Hooks, 3M Command Industrial Strength Velcro, or some other type of hooks to mount the tube fixture under the lid, and the T8 tube will need to be within 8" of his basking spot/platform at the furthest away, so the UVB tube will unobstructed, with nothing between the tube and the beardie. You cannot mount the Reptisun 10.0 T8 UVB tube on top of a mesh lid like most enclosures have, as the mesh blocks most of the
UVB light (glass blocks ALL
UVB light, none gets through glass, just as an FYI).
What I suggest you do since he will most likely be in the hospital for a few days and you have time to have an order ship, is you go online to Amazon.com and order the Reptisun 10.0 (not 5.0, remember this as some cheaper packages have a 5.0 tube, which is for tropical reptiles only) T5 High-Output UVB tube. The Reptisun 10.0 T5 High-Output UVB tube is the best
UVB light you can buy for a beardie, and is excellent for a beardie who has a calcium deficiency or MBD. The 10.0 T5 High-Output UVB tube can be set on top of a mesh lid (not glass) because the
UVB light is strong enough to penetrate the mesh, as long as the tube is 11" away from his basking spot/platform at the furthest. Also, the T5 needs replaced once every year, while the T8 needs replaced every 6 months (they stop emitting
UVB light at these times even though they still turn on and emit light). If you go to Amazon.com and search "Reptisun 10.0 T5 HO UVB Tube" you'll see different length options, and once you choose your length (either 16", 22", 34", 46"...The T5 High-Output tube is listed in weird lengths, but the 16" fits in an 18" fixture, the 22" fits in a 24" fixture, etc), you'll see a package deal presented to you with the 10.0 T5 HO UVB Tube and a matching length, T5 compatible fixture for a great price. If you were to buy the Reptisun 10.0 T8 UVB tube in a pet shop, you can actually go to any Walmart and go to the aisle with lightbulbs and flourescent tube lights, and buy either a regular 18" or 24" long flourescent tube fixture for around $10 that is for a T8 strength tube, along with the 3M Command Hooks or Industrial Strength Velcro to mount the T8 inside the enclosure. You won't find a long flourescent tube fixture that is rated for a T5 strength tube of 25 watts like the Reptisun 10.0 T5 is.
The only other light you need is his daytime basking bulb (doesn't need a nighttime heat source if your house stays at 65 degrees or above at nighttime, they like it cooler at night just like us). All that you need to buy for a basking bulb is a regular, household halogen indoor flood bulb that is bright white, as most are. The ordinary PAR38 indoor flood bulbs are great and only cost a few bucks. No need at all for a special reptile basking light that you buy at a pet shop. If you'd rather buy one at a pet shop, I recommend the ZooMed Intense Spot bulb, it's bright white and comes in several different wattages for around $10-$12. Absolutely no colored basking bulbs, not blue, red, green, yellow, none, only bright white, which is why I like the ZooMed Intense Spot bulb, they only come in bright white, and they come in a black and white box. The wattage of your basking bulb is going to depend upon the size of your enclosure, but figure if you buy a 40 gallon breeder tank, which is the minimal size for an adult beardie, you'll want either a 75 watt or a 100 watt bright white basking bulb. You'll also need a regular clamp lamp or a single dome lamp for the basking bulb. That's it, the UVB tube light and the single basking bulb are the only 2 lights you need.
Also grab a digital probe thermometer with a probe on a wire at the pet shop, they cost around $9 and are the most accurate way to measure temperatures, and really the ONLY WAY to measure the basking spot temperature.
Once you get this stuff we can help you with adjusting the lights/basking area and getting the temps correct...He's probably not going to be able to move around very well, or not at all, for a while, so I wouldn't buy any decor that is difficult for him to get onto to bask, or that he has to climb too far up for now..