What are you using for an enclosure? You said he is "practically on the UTH". You don't actually have the uth in the enclosure, right? It should be on the outside of the glass or tub (depending on what you are using).

What is the temp directly on the surface above the UTH (given that you have the uth placed properly)? It should be between 88-94 degrees. If it is on the lower end of those numbers, you might want to bump the temp up -- he might just be trying to get closer to the heat source because he wants to be warmer.