The back is the safest position due to the risk of SIDS. But babies move around as they get older!
Here's information from the WebMD Health & Baby Center:
As babies mature, they learn to roll from their backs to their stomachs. Babies who roll onto their stomachs during sleep do not need to be continually shifted onto their backs. However, always initially place them to sleep on their backs.
Ahhhh..sleep issues! (I'm still having them with my 2 year old!)
A few questions: What time is he put down to sleep at night? What is his nap pattern during the day? Are there any outside noises that could be causing this? (Traffic, a loud bird by the window, heater coming on, pets in the house)
Also, how is the waking handled? Is he allowed to Cry It Out or is he picked up, etc?
Let me know a few more details and more members can chime in with suggestions!