Hello, im thinking you may want to bring this guy to a vet. If all your husbandry is spot on and nothing has changed for example the amount of day light and night he gets something else has to be up. I would give his normal enclosure a good cleaning and put him back in there until you figure out what is wrong. The bare tank is a good idea but he has no other cage mates to spread anything to if he does have something so you could put him back in there for the time being. I would put just enough eco earth that he can burrow down abit but you can still see him. The reason im saying this is because they burrow to cool down and get out of the direct sunlight, but they also do this to feel secure and safe. If I start fiddeling in my pacmans enclosure he will totally bury himself or if hes in a bad mood he will try to attack me. If he isn't able to bury he will become stressed and that will lower his immune system and could just make what ever is going on a lot worse. Again I think its time to bring him to a vet and if he hasn't eaten in like 2 weeks you may want to try and force feed because he is looking thin.