If he is strong enough to turn himself over, he is strong enough to pick up his head if he has trouble breathing and even cry. Keep a monitor in his room and a reciever with you so you can hear him if he does get into trouble and try to relax the best you can.

I felt the same way with Mimi until she was 6+ months. By then she was pulling herself into a sitting position and I knew for sure she could let us know if she were in trouble.