My Big Roller

So last week Derek started rolling over. From back to stomach and from stomach to back. I would put him on his play mat to go and get something and when I got back he would be across the room! Well last night I put him to bed and about five minutes later I felt like I should go and check on him... what did I find my little roller on his stomach asleep (not just on his stomach but face down). So what do I do? Leave him or roll him back? Any advice? (this picture was not taken until the next morning and I know he is not really on his stomach here, but last night he was)