Get a thermostat asap, or in the meantime if you need to order one either unplug the UTH or get a lamp dimmer from a hardware store and plug the UTH into that. A thermostat will regulate the heat pad so it doesn't overheat.

As to if it's already caused damage: if he doesn't have any burns/pinking on the belly not associated with shedding you probably caught it in time. So either unplug the UTH or get a dimmer and look for a good thermostat: I like the VE200s but they're $115, but there's one on Amazon for about $30 that many people use and like.