When is it safe for belly sleeping?

My little guys rolls from back to belly and belly to back. I always put him to bed on his back but the past few nights he has been rolling onto his belly to sleep. If I rolled him back onto his back he eventually rolls back onto his stomach. He's officially 5 months old today. My question is, is it okay for him to be on his belly? I'm a FTM so I worry since they say not to let them sleep on their belly until they are over a year old but he won't stay on his back!