Baby sleeping on stomach

My just turned 5 mo old can roll himself both ways and has been able to for a while, now he is almost always rolling himself onto his side or belly to sleep. It’s safe for him to sleep that way if he does it himself right? I always put him down on his back but often he turns himself right away. He sleeps in a safe space and I watch him on the monitor for naps, he sleeps next to my bed in his own at night