Your humidity should be fine unless you're running an A/C all the time. You can always put a container with damp moss or vermiculite or paper towels in it for him to use.

He doesn't need anything higher than 86, though it is OK if it goes higher. Don't worry about the lows, if he gets down in the 50s he'll still be ok. I doubt it gets that cold in your house.

A picture would help. Sometimes desert kings aren't really desert kings.