Flipping over while sleeping

I know they say that once a baby can roll onto his stomach he should be safe to sleep that way. We put my son to sleep on his back only. Today he rolled onto his belly and fell asleep again. The only reason this makes me nervous is because he has mastered back to belly rolling but hasn't rolled from belly to back. I know that once he can roll he's strong enough to reposition himself if he needs to but it just makes me nervous. Is it okay to leave him that way if he isn't able to roll back? It's so weird because he hates being on his tummy lol.