I believe you should have your T-stat probe on the outside directly on the UTH. Then have your themometer probe on the inside where his belly will be. Then adjust your T-stat to get the desired temps on the inside of the tub according to your thermometer.
Try replacing the batteries also.
And remember, our body temp is high 90's, so if its barely warm to you, it's probably in the 90's.
I'm just about to get that thermostat and I've heard good things, and people on this forum who have it also like it.