Losing sleep because your former back snoozer is now flipping onto his tummy? You probably don't need to risk waking him by trying to roll him back. If your baby can get himself onto his tummy, he can usually get himself off it, because rolling from back to tummy is the tougher milestone (try it yourself and you'll see what we mean).
By the time your baby has mastered rolling around, the SIDS risk has dropped dramatically (the peak risk is at 2 months, with a steep drop from 3 to 4 months). You can start to breathe easier with the knowledge that your baby is growing stronger.
The key—as with just about everything else in parenting—is to be consistent. Continue to put your baby to sleep on his back (and make sure any caregivers do, too), since it's still the safest method. Don't bother trying to use a sleep positioner either: According to the American Academy of Pediatrics, there isn't sufficient evidence on the safety or effectiveness of such devices. Also be sure to steer clear of suffocation hazards in the crib, such as stuffed animals, comforters and bumper pads.