Help!

My almost 4 month old has learned to roll onto his belly. We have stopped swaddling at night because of this so he can push himself up. However, when I put him down last night he rolled over on to his belly, face down. I gave him a minute to see what he would do and he never turned his head to the side. He just stayed face down. Now I can't sleep because I'm worried he's going to suffocate at night. Help, what do I do?!? Any suggestions for me?