M.D.
I assume he can roll over on his own? If so, I wouldn't worry about it. I was always told that you put babies to sleep on their backs, but once they can roll over, they will sleep in whatever position they prefer. Just keep the crib clear - no pillows, fluffy bedding, etc.
No doctor expects you to force a child to stay in one position. If he can't roll over, I'd probably keep putting him down on his back, but once he can roll over on his own, he'll do what he wants.
I have heard a lot of negative things about the sleep positioning things and I believe the latest research says they do not help prevent sids and may even pose more problems rather than help.