All heat should be regulated by a thermostat. You can get one online made by Hydrofarm for like $40.00. Unfortunately reptiles do Not have the capacity to move when they are getting burnt. They don't feel it. Therefor we need to prevent it. It sounds like he isn't getting warm enough on his belly. That's the heat they really need to digest their food and regulate their temps. I would make these changes immediately.
- get rid of the heat lamp get an uth that covers no more than 1/2 of the tank
- get a thermostat
- put digital thermometer probes on the floor of The hot side and cold side. Temps on hotside should be 92.f and cold side 80.f

Hopefully this helps you