Night hat? Help!

I need help! My dear son has been sleeping like this for as long as I can remember...and I have thought many times that he needs a sleeping hat! I asked him the other day if he would wear one, if I sewed him one. He said yes. So I went and searched through my few clothing patterns, nothing there. Searched online, but haven't found much we like. I don't knit, well not very well, and besides, Dragon says the wool would probably make him itchy. I thought of polar fleece, but not sure if that would be too hot...With cotton, an elastic would be needed I imagine, but that wouldn't be comfortable while sleeping...I am not sure what else?
I would love some help on this, and to not worry anymore that my son might tangle himself up too much in his bed sheet. What did boys or men use many years ago? Please leave a comment if you have some help or links for me! Thank you from Dragon and me.

ps. I did sneak and take that sweet picture of him while he was sound asleep...aren't they just so cute when they're sleeping?