Leopard gecko heating?

My 3-4 month old leopard gecko has been staying in his humid hide alot, and if hes not in there hes laying his belly on the outside of his warm hide (more to the cooler part of the tiles). He has a healthy appetite, hes active, hes not shedding and the temp on his warm side is 88-90 degrees during the day and I turn it off at night and use a infrared bulb and the temp is usually 80 at night. I have a UTH and as my substrate I have slate tiles over sand. I thought maybe the tile under his hide was too hot so I cut a paper towel, folded it, and put it in his hide. But he still won't go in his hide. Why is this? Should I add more sand at the bottom? With the paper towles in the hide its about 87-88 degrees so its not too hot. I just don't know what I can do I just want him to get the adequate heat he needs, is it normal for leopard geckos to stay in their humid hide alot I heard some do, so any help would be great! Thanks!