D.L.
Well, once a child learns to roll over one way it's not long until they can roll over how ever they want. I would just wait it out, he'll probably take longer to learn to roll back onto his tummy if you do it for him. My kids both slept on their tummy as babies and still do today. In fact, when I'm trying to get them to go to sleep I know they're just about to calm down enough to go to sleep when they decide to roll onto their tummies.