I would recommend taking your thermostat probe and adhering it to the outside of the UTH (on the bottom). I recommend this because then your snake will not be able to move the probe and give your thermostat a false reading. If this happens, the UTH can overheat.
I would also take out the reptile carpet. That stuff is very hard to clean. If you really want something in between the glass and aspen you can use paper towels. Cheap and disposable. Don't rely on the substrate to keep your snake off the glass, always assume your snake can get to any point within the enclosure. To ensure no place in the tank is too hot, I like to place a probed thermometer (separate from the thermostat) right on the glass above the UTH, under the substrate. This will give you the temp of the hottest point the snake can reach.
For now, I would just keep the snake on clean paper towels until you can figure out whats going on with his belly.
Did you purchase this snake from a pet store? I know a good breeder would never sell an animal to someone in that condition.Is there any reason why you purchased it from the place you did?