It sounds like belly rot, or possibly a burn from a uncontrolled under tank heater (UTH), it is not a bad shed.
Firstly he needs a vet ASAP as from your description he may well need antibiotics.
Secondly if you have an under tank heat source that is not connected to a thermostat then unplug it right away.
Thirdly we need info on your set up so we can help with your husbandry - tank/tub, size, temps, heat source, humidity, substrate.