If you really want to sleep him on his tummy, and he sleeps better that way, and you feel comfortable doing it, then go ahead. For ages and ages babies were slept on their tummies, and most came out okay..
If you would feel better though, you can buy a breathing monitor, u put pads under the sheets on the mattress, and sleep your baby on it, and if for any reason the baby stops breathing for more than a certain time (think its 15 seconds) it sounds off an alarm, that will both let you know, and jolt the baby which will make him breathe again. They are pretty expensive, about 200 or so, but you can buy some off ebay for cheaper im sure. Something to look at any way.
Maybe, you could sleep him on his side, have yout ried that?? does it make any difference?