5 month old and sleep

My 5 month old has been turning in his sleep and ending up on his tummy with his face either in the sheet or turned to the side. I wake up and check on him all the time and he is always on his tummy even though I place him on his back to sleep. He can sit unassisted for 15 seconds and holds his head up well and can turn over the second I lay him on his back for playtime. He has good strength and mobility of his trunk and neck. Is he ok to sleep on his tummy? They say once he can turn his head it's safe. I am just scared to because of sids.