Unfortunately I'm not sure I can achieve a better gradient without raising the warm side temperature. The room he's in is just a bit on the warm side in the summer. I'm working on a way to cool the entire upstairs of the house, so maybe he will benefit from that.
However.. he has outgrown his hide a bit. I will get him a new large one for his cool side and if he takes to it, a similar one for the warm side. I'll also leave him alone for a good while and see if he will eat.
Thanks for the advice, guys.
Edit: Oh, and I've had him for uh.. mmm.. 1 year and 3 months. Haven't weighed him recently.