Rolling onto belly at night

My son has been sleeping in his crib since he was 3 months old and he has been sleeping great. But now that he can roll from back to front and vice versa, he is starting to roll onto his belly in his sleep. He will be 6 months in a few weeks and he is capable of holding his head up well, but I'm a bit worried. Last night we flipped him back onto his back but he just rolled again lol. Should I be concerned, or is he okay at this stage?