Belly sleep

My 5mo can roll from back to belly, can do tummy time with head up for really long periods of time, and can sit unsupported for about 5-10mins. He can also pull himself around (like an army crawl) on the ground. But, he can't - or just doesn't- roll belly to back yet. Lately he's been rolling over to his belly in his sleep. So far, his nose has always been exposed and he even switches his head position.... But I'm still paranoid! Do you think it's ok for him to sleep on his belly? Should I just let him do that without flipping him back?