How to get my baby to stay asleep.

My 5 month old insists on waking up every 3 and a half 4 hours throughout the night. He doesn't take long naps during the day and we even put cereal in his night time bottle. He tosses and turns a lot. Sometimes he stretches and ends up rolling over (he's a belly sleeper) and when he's on his back he wakes up. He used to sleep through the night and he's puts himself on a 9-9:30 bed time. How can I help him (and myself) get more sleep?