K.,
There is a co-sleep device that keeps them from rolling. It is rather expensive. Babies R Us carries it. I wasn't sure if you were saying he co-sleeps or he is in one of these devices.
I don't know where your son sleeps. The bedclothes to keep me warm are pretty heavy, so I have to put my son near the head of the bed, and it doesn't matter where he rolls from there.
He is staying more and more in his crib now, though. For my son, I think rolling over was a comfort/warmth issue - he only slept on his back really well during the hot months. As soon as he could roll over when it was colder outside, he was curled up on his tummy.
You might check the thermostat/his skin temp when he does roll over.
When I addressed the pediatrician (and really it was the nurse at the time), you start them off on their back. If they roll over in their sleep while you are sleeping, you can't really do anything about it. She didn't go beyond that.
If it would help you be less scared, look up the statistics on SIDS by age group (and it goes to age 10, but that's extremely rare).
Good Luck! Scary issue I know, but you can't spend his infancy being scared!
M.