So are you currently using the heat mat without a thermostat? If so, unplug it until you get the thermostat. You don't want unregulated heat mats on at any time, but especially not when his belly is already irritated. If his ambient temp is 80-85 going without a heat mat for a few days won't hurt a thing.

When you say it is looking worse, what do you mean? Does the affected area look bigger, texture change, color change, ect.?