Once a baby can roll over, they pretty much pick how they're going to sleep, and there's not much you can do about it. My oldest rolled over at 3 weeks (very young, I know), and slept on her stomach from then on. As long as you keep the crib empty of loose bedding, stuffed animals, and pillows, and the baby is able to move his/her head, you should be fine. If you are worried about him being cold at night, you can buy a sleepsack (or blanket sleeper, I prefer the ones without arm covering). This will also eliminate the possibility of him getting stuck in a blanket.