doesn't know how to turn back

My son is 6 months old. He knows how to turn on his Tummy but doesn't know how to turn back. Every night he falls asleep on his tummy but wakes up crying 45 mins after, I guess he tries to turn but doesn't know how to. This happens every single night. I put him down on his back. Anyway to avoid this from happening? I don't want him waking up