Tummy sleeping baby

So every night this last week,  I go in and check on the baby and he is sleeping soundly. I place him on his back to sleep (per the AAP) to prevent SIDS. I find him asleep on his tummy his little head cocked to the side. I flip him over before I finally go to bed again and I wake up in the morning and he has managed to flip himself back on his tummy again. I guess I can give up trying to get this stubborn little baby to keep sleeping on his back... So wierd. I thought they weren't supposed to do this until they got much older.