Hi...I'm having the same issue with my DS (almost 4 mths)...he's rolling much earlier than DD did and I'm always finding him on his tummy. I always place him on his back in bed but he will either roll onto his side or completely flip over.
So I can't really answer your question, I guess you could just let him be. Where does he sleep? My DS cot is right next to me in my room and will be there for a while yet, it's supposed to be the safest place for young bubs to sleep. You could get an angelcare monitor that senses when bub stops breathing for more than 20secs (I think), these are obviously not failsafe but could set your mind at ease a bit.
[Edited on 21/12/2009]
Charlie girl 15/01/06 & Dean the Machine 22/08/09