I have five children. My first I tried to keep on his side, as was en vogue then. My second I tried to keep on his back. I gave up those ridiculous efforts--all of my children slept on their tummies. They are healthy, happy, and have no sleeping problems. Once, the other night, my five 1/2 month old son (June 10th for me--congratulations!) rolled so much he woke me crying that his face got covered by the blanket. (forehead and eyes) so, I dressed him warmly and took the blanket out of the crib. He was fine.
Your son will be fine. Let him sleep as he's comfortable, just take suffocating things out of the crib. A blanket is okay, too, just make sure it's not too heavy or too high on him.
Relax! :)
~A.