Hi, welcome to the forum...I'm sorry your guy isn't doing well, the good news is that I don't think he's sick at all, the bad news is I think it has something to do with either his lighting, temps, diet, or a combination of all of these.
Can you please post some photos of him, his entire tank and set-up, including his lighting? That's always extremely helpful.
Also, what
UVB light do you have (brand/wattage or strength/type). By "type" I mean compact, coil, long fluorescent tube, or Mercury Vapor Bulb. Also, how old is your
UVB light right now? How far away in inches is your
UVB light from his main basking spot/platform? And do you have your
UVB light on top of any type of mesh lid? If your
UVB light is a long, fluorescent tube, is there any type of clear, plastic cover on the bottom of the tube fixture, over the UVB tube?
And then your 3 temperature zones, can you list all 3 current temps? (Basking Spot Surface Temp, Hot Side Ambient Temp, and Cool Side Ambient Temp)? What type of thermometer do you use to measure his temperature zones? (stick-on, Digital Probe, or Temp Gun)
How many hours per day do you leave both his Basking Bulb and his
UVB light on? What wattage/color of Basking Bulb do you have?
It sounds like he's most likely around 9-10 months old, as they are typically between 1-2 months old at Petco and Petsmart. So he's still a juvenile. He's definitely large enough to eat superworms, but superworms should not be his daily "staple" insect, superworms should only be a Bearded Dragon's supplement to their actual "staple" live insect, or an occasional treat. They are very meaty and so if you're feeding him size Medium or especially Large Superworms, you have to realize that 2 Large Superworms is a meal, and is way too much fat. That's the problem with feeding Superworms as their "staple" live insect, they have a low protein content and an extremely high fat content, so often beardies who are fed Superworms daily end up with Fatty Liver Disease and live much shorter lives than those fed appropriate live "staple" insects that are high in protein and low in fat, such as Crickets, Roaches (several different species are appropriate), Silkworms, or Phoenix Worms/BSFL/NutriGrubs/ReptiWorms/CalciWorms (all the same thing). For instance, I have been feeding my Beardies size Large BSFL as their live staple insect for the past 10 years, and I will supplement 1-2 Large Superworms only AFTER they eat their fill of BSFL, so they usually will only eat 1 Superworm or none because they are full, and I only offer them every other day. So I suggest choosing a healthy staple live insect and ordering them in bulk online from a place like
http://www.dubiaroaches.com or one of the many other places that sell all live feeder insects, in bulk, extremely cheaply.
If you post some photos and answer the husbandry questions, we can make sure that there's nothing going on that is negatively effecting his health. I wouldn't worry about him only pooping "every 3 days", as they do tend to poop less often the older they get. Some dragons only poop once a week, or once every 10 days, and this is completely normal, so that's not necessarily an issue, and I'm not thinking that it is in this case either, because he is approaching a year old. The other thing to keep in mind is they do usually have a normal "Slow-Down" period that typically happens between 6 months old and a year to a year and a half old, where they do become less active, they do begin to eat less live insects and should be in-turn eating more fresh greens/veggies. So if you're not offering him a nice, big, fresh salad every day now, made up of healthy greens and veggies, then you absolutely need to start ASAP, as he's approaching the age when they do start the switch to less insects and more salad. He still needs to eat a large amount of live insect protein every day until he's at least a year old, as he's still growing, but he may just be transitioning out of being a juvenile and heading into being a sub-adult...
That being said, it's always a good idea to go over your husbandry to make sure it's all good...