My best advice is to get a waterproof cover that completely encases the mattress you buy for him. Not only will it completely protect the mattress from accidents, it prevents dust mites, etc, as well. I have plastic zip covers around all our mattresses, then covered by a nice cushy mattress cover...can't even tell they're there, but they've saved our mattress.
In fact...had a vacuum salesman trying to "scare" me, I guess, into buying his vacuum...he asked if he could vacuum my mattress. I guess his point was going to be to show me all the dust mites, etc...pad came off CLEAN. Yay!!!
My oldest has sheets on her bed and a comforter, and uses neither. My youngest (20 months old) uses sheets AND a blanket. Everyone's different...you'll have to experiment. :)