Beardie not eating much, very lethargic

Our beardie was passed on to us over a year ago from at least 2 families, so we have no idea how old he is (or if it is even a boy - the vet couldn't tell us!). He has only ever eaten very little, but now he's down to only a Tbsp or so a day (if that) of veggies/fruits. He's very, very skinny, with baggy skin. He will eat any moving insects we give him, but I was trying to limit those in the hopes that he'd eat the veggies. Should I just give him all the crickets he wants? Our UV light was only bought 4 months ago, so it should still be fine. Heat seems to be ok - somewhere between 80-90 degrees. Cage is clean, he's pooping normally (once a day at least). He's always been a super chill beardie - not a lot of movement. Now he rarely moves at all, unless he's chasing a cricket. He does have a bump inside his lip which the vet believed to be the beginnings of mouth rot. He gave us antibiotics to give him for 5 days. No change in the bump, but nothing worse has developed. Vet has suggested cutting off the bump and cauterizing it, but that seems really fraught with possible problems - infection, etc.
We love our beardie and don't want him to die if there's something we should be doing for him. I've been dusting his food and insects with calcium and multivitamins. We do take him out often and carry him around, so he gets a lot of love. Ideas? I don't think the vet has any other ideas. I'm just not sure how to entice him to eat more.

We just changed him off of a sand substrate to a carpet and feel that's much cleaner.
He probably eats 6 crickets a day and about the same in small mealworms. Should I be offering more? We rotate his fruits and veggies from an online list I got off of a bearded dragon site.

Previous to the last few weeks, we weren't feeding him many insects or rotating his food well because honestly, I wasn't as engaged in his care and had kind of delegated it to my son. Now I'm trying to get back up to speed and figure out what might be going wrong.

See bump in mouth.
Vet says it is scar tissue over an infection.

have you tried a variety of greens? basking temp? and stop feeding mealworms they are high in chitin which causes impaction feed superworms instead. He doesn't look skinny to me

Hi. He doesn't look too bad. What kinds of lights and brands? How do you take the temps? Do you dust his feeders with calcium and vitamins? How close can be get to the lights? What size is the enclosure?

Is there a warm and cold side?
You can try measuring your beardie to estimate their age. Try a variety of greens like mustard, turnip and collards. Also a variety of vegetables like butternut squash, yellow squash and bell peppers.
I reccomend trying super worms instead of crickets. They are better for them and will be able to hide in the greens and entice your dragon to eat the worms and snag some greens. Usually when they accidentally snag greens they find the taste okay and return after some time and training with the worms.
Keep the greens fresh, replace twice a day even if that means tossing some (or gut loading your worms with the dried greens).
Do you have a water bowl? Beardies drink water and absorb it in their skin, so keep it clean since they'll take a splash in there.
I hope this helps