You could turn it back up a degree or two if you wanted to (back to 90), but not much more than that. If he wants more than that, he'll rearrange as needed to find the heat. The most important number is going to be that tank surface temp. That, combined with a thin layer of substrate will be enough to ensure proper digestion.

It sounds like you're being appropriately cautious as a new owner. They are pretty resilient little buggers, so make sure you're having fun with your new addition and not worrying yourself too much!