if he doesnt wake when he rolls over then I would just leave him wheere he is even if it looks uncomfy to you - if it was that bad he would wake up anyway. Although ds always ends up on his tummy I always put him to sleep on his back in the first place - I think that is the advice given on the SIDS website:
My baby keeps rolling onto his front. What should I do?
If your baby is less than six months old and you find him sleeping on his tummy, gently turn him onto his back. Don?t feel you need to keep getting up all night to check on this. Whatever your baby?s age, always place him to sleep on his back.