Wool fleece is used to decrease skin breakdown (pressure sores), so any slipper lined with real fleece (or boot) will help his heels. I began having heel sores fairly early on and at that time (problem only from bed, I was still able to stand somewhat), used synthetic fleece heel cups in bed. Now I wear Uggs (sheepskin), sleep on wool fleece in bed and also have my wheelchair arms, seat and back covered with sheepfleece. It's less of a problem in the summer than you'd expect since wool wicks perspiration and "breathes". See the "Tips, Tricks, and Gadgets" area...source is found there if you're ever interested in trying it.
This is indeed a very common problem, whether in bed and unable to turn or in a chair---Bad Balance is right!
I stay in airconditioning during the heat of the summer---but do go out on the deck for some sun.