I had the same problem with my ds at that age. He was also a tummy sleeper (still is), and would wake himself up when he flipped over onto his back and couldn't get back to his stomach. The best thing to do is lots of tummy time during awake time, so he can master this new skill. This way he can learn how to roll back over, and the more opportunity he has to practice, the quicker the novelty of practicing the new skill will wear off.
Good luck!