I know babies are supposed to sleep on their backs, but mine keeps turning over on his tummy. What should I do?

My baby did that when she was about 3 months. My hubby and I became light sleepers during that time and would jump out of bed periodically to check on her. We didn't want to wake her as she slept better that way. Gradually we became more slack and she's 6 months now. Just make sure that there aren't any pillows, blankets or soft toys in case of suffocation.