He's not going to perk up or start eating until you get your 10.0 T8 UVB tube UNDERNEATH THE MESH LID!!! Seriously, the mesh blocks 40% of the UVB/UVA light emitted by that T8 strength UVB tube, and unfortunately a T8 UVB tube is the weaker of the two types available (T5 being the much stronger), so #1) You can not afford to have 40% of the
UVB light blocked from a T8 strength UVB tube, he's still getting essentially NO
UVB light!!! You can poke 4 holes in the mesh and use some of those long, plastic Zip Ties to attach the entire fixture to the underside of the mesh, OR JUST GET RID OF THE MESH LID ALL TOGETHER OR PUSH IT TO THE SIDE AND REST THE UVB TUBE FIXTURE ACROSS THE TANK!!! Be sure to keep both the UVB tube and the bright white basking bulb right alongside each other and both directly over his basking spot. Moving that mesh lid over a bit to give him direct, unobstructed
UVB light is not going to effect the temps. AND PLEASE BE SURE THAT BOTH LIGHTS ARE ON FOR AT LEAST 14 HOURS EVERY SINGLE DAY, AND ARE ON FOR AT LEAST 2 HOURS IN THE MORNING BEFORE FEEDING HIM, AND AT LEAST 2 HOURS IN THE EVENING AFTER YOU FEED HIM, SO HE CAN DIGEST HIS FOOD!
I wouldn't expect him to perk up and become active OR gain an appetite and start eating his live insects until you give him 14 hours of unobstructed
UVB light, and the UVB tube at no further away from him than 5-6", 4" is better with that T8 UVB tube. I know it seems like such a petty little detail, but you must realize that bearded dragons are desert reptiles that get strong, intense, bright UVB and UVA light all day long every day in their natural environment. This is absolutely crucial to them being able to manufacture their own vitamin D3, which in turn allows them to absorb and process/use any and all nutrition (calories, fat, carbs, protein, etc.) and any vitamins and minerals, including calcium. So quite literally, without him getting 14 hours of unobstructed, direct UVB and UVA light every single day he is not manufacturing any vitamin D3, so in turn he's not been absorbing or processing/using any of the nutrition or vitamins/minerals that you've been giving him or that he's eaten on his own, he just excretes it out. So if he's been absorbing no nutrition, he certainly will be lethargic, weak, and not have any appetite at all.
For now just move the mesh lid off to the side, as AHBD already mentioned, exposing the unobstructed UVB tube to him, and making sure you arrange his tank so that his main basking spot/platform is directly underneath BOTH the unobstructed UVB tube AND the basking bulb (they must be right next to each other), as this is how you duplicate natural sunlight as closely as possible for him, so he gets both UVB and UVA light, along with other essential UV rays, and heat. Also be sure that his main basking spot is putting him with at least 5", as I said 4" is better, UVB tubes don't emit any heat so he won't burn himself, and a Reptisun 10.0 T8 UVB tube will not hurt his eyes or skin, so the closer you can get him to that unobstructed, exposed UVB tube the better. Allow both lights on for 14 hours a day, and about an hour or two after you slide the mesh lid to the side and set up his lighting correctly with the
UVB light not being blocked anymore, use your probe thermometer to check the main basking spot surface temperature again, just to make sure it is still between 105-110 degrees...BE SURE TO LET THE LIGHTS IN THE NEW POSITION WITH THE MESH LID PUSHED TO THE SIDE FOR AT LEAST AN HOUR TO LET THE TANK GET TO THE TEMPS IT WILL NOW BE, AND ALSO BE SURE TO ALLOW THE PROBE TO SIT ON THE BASKING SPOT FOR AT LEAST 20-30 MINUTES BEFORE READING IT. If the basking spot surface temperature is no longer between 105-110 degrees, simply lower the basking bulb down a bit, let it sit for another 30 minutes to an hour, then put the probe back on the basking spot for another 20-30 minutes and check it again.
In addition to the fact that the mesh lid has been keeping him from getting adequate
UVB light this entire time, you also have to remember that that vet put him on a very, very harsh medication that was not only completely unnecessary and a dangerous decision to make on just a hunch or a guess, unfortunately the vet chose the worst medication possible to give him. It's a horribly strong drug that completely knocks their entire gastrointestinal tract out of whack, depletes all the healthy bacteria that normally lives in their guts, and most harmfully it puts enormous strain on their kidneys. Not to mention that even if he had actually done a fecal test and found a high count of Coccidia, #1) That is not the medication that ANY experienced reptile vet would ever prescribe unless they had done an additional culture and sensitivity and determined that the strain of coccidia that he had was one of the only two strains that respond to a sulfa drug, and #2) Unless the amount of coccidia he found was "to high to count/detect" he should not have given a baby dragon that small any medication for parasites, not if it was a low or moderate level.
So the bottom line is that your little guy was done a great disservice by that vet, had he been an experienced reptile vet he would have asked you about your husbandry BEFORE doing anything else, and the minute you told him what
UVB light you had, how it was mounted, and how low his temperatures initially were (and how small a tank he was in) the vet would have immediately known what the problem was and helped you to correct it. But that's not what happened, so unfortunately it is going to take a while for his gastrointestinal system to feel normal again, some probiotics may help him a bit, but I'm willing to bet that a day or so after you get that mesh lid out from under the UVB tube and get him within 4" or so of the unobstructed tube, finally your beardie will be getting adequate and strong enough
UVB light for a long enough photoperiod ever day, and he'll perk right up and start eating on his own in a day or so.