All 3 of my sons slept & still sleep on their stomachs & as long as you don't have all toys & blankets all over the bed I wouldn't worry about him not being able to breath. Babies have a thing for comfort & when they not happy or can't breathe properly they SCREAM!!!
Keep him in a long sleeve/pants PJ for the cool air at night.
OR just layer him with a light cover such as a recieving blanket. that is what my babies always loved.
Try placing him on his side with a recieving blanket rolled up one in back of him slightly under his butt & one in front. It gives them the feeling as if someone is holding him. It's back to that comfort thing Babies LOVE!!!