You've probably been told many times that babies should sleep on their backs. But now that your baby is rolling over, you can't control his sleeping position.
It's probably OK to let him sleep in whatever position he finds most comfortable. Once a baby can roll over on his own, generally between 4 and 7 months of age, the risk of SIDS (Sudden Infant Death Syndrome) is usually reduced.
Keep thick blankets, comforters, pillows, and large, soft, stuffed toys out of your baby's crib.