Oh J., I know exactly where you are coming from. 1 of my twins boys (2 now) did this around 4 months & it literally terrified me too. I too stayed up for hours on end continually flip him back over, time & time again. It was pointless. He wouldnt have it. He kept flipping & i kept flipping him back for hours....Finally, I just thought that it was hopeless & he was just determined he wanted to be on his belly. I prayed over him & asked that the Lord protect him & send angels to watch over him while he slept & prayed for the Lord to wake me if they needed help. He is now a rowdy 2 yr old who still sleeps on his belly. My point is that i had the same fears of him blocking his ability to breath or of him rebreathing exhaled air vs oxygen. To this is a truly legimate concern, but when they are old enough to start flipping over, then he most probably is able to get himself out of a situation. Having said that, you as his mother have to make the call & use your own motherly instinct to make the end decision whether or not to try & make him sleep on his back. Personally, i struggled with fear alot when it came to my twins - but fear doesn't come from the Lord. I have learned to cast my fear on the Lord (although that is a daily effort). I will tell you too that both of my boys have special blankets & on numerous occasions I have gone in to check on them & they have their blankets over their heads & that scares me (they didn't have them in the crib until older though). But they are still fine. I do understand your concerns, especially since your baby is still so young. Do what you think is right, whatever that may be.
**make sure the sheets are tight
**make sure he has on warm pjs
**no pillow or toys or stuffed animals
**mine didn't have blankets until they were older, i just dressed them warm enough in pjs - if you wanted a blanket you could get a swaddler & just swaddle him starting under his arms & down. We did this with the other twin because he liked his arms out.
Bless you,
V.