What is your temp on the warm side and how are you regulating the heat source? Also, are you using a UTH, heat bulb, or both?
My guess is if he is staying on top of the warm hide, then it is either too hot inside and it's cooler on top, or, if you are using only a heat lamp, then it is too cool inside and warmer on top. Or possibly he made a mess in the hide and doesn't want to go back in...
Rob