My son is being bitten in his sleep by something?

My son, 17 months old, is waking up with more and more bites on him. Ive searched his crib, replaced all bedding, cleaned everything like crazy, and something is STILL biting him...They are raised bites..very red, and the most recent one and two holes (like fang marks). Ive never seen a bug in this house before, and I'm really upset that I cant find whats doing this. I was thinkng bedbugs, but his crib mattress isn't like ours...its got a plastic type outer cover. I didnt figure bedbugs could live on it...Someone PLEASE help me, it kills me knowing I cant help him!