My 6yr old son has this problem along with my 2 1/2 yr old daughter, they both get really hot when sleeping. For my 6yr old, sometimes it is just his head and at other times his whole body. My 2 1/2 yr old always wakes up with a sweaty head from her naps. She still sleeps in her crib, she is normally wearing shorts and a tshirt and never can leave her blanket on her. I try to keep her fan on to circulate the air. At first I thought it was the fleecy puppy pillow that she slept on, but now she has a regular pillow with cotton pillow case, and she still wakes up with damp hair.
First thing I would look at is what he is wearing when sleeping, what he is sleeping on and what type of covers he has.
I have even noticed that when my kids are sick they heat raises up and they have sweaty heads..