Rolling over fears

So my baby can rollover on to his belly. This so cute and fun to watch! What frightens me is that I have been finding him asleep on his belly during the night, and naps and that he can't breathe. I can't even remember how many times I got up to turn him on his back last night. His head has always been turned to the side not straight down on the mattress (he sleeps in a crib in a crib in my room). Any suggestions or advice is appreciated!