K.V.
Personally, I would just leave him be when he rolls. My son has been rolling in bed for the last couple of months and I just let him be and he gets himself comfy and goes back to sleep.
My rule of thumb about sleep positioning is this...if they are comfy, let it be. I had a back sleeper and a belly sleeper. My belly sleeper sleeps much better than my back sleeper did. I was very concerned with my first child and the sleep positioning so I made her stay on her back all the time...which I believe led to her not being a good sleeper. My second child I let sleep how he was comfortable...and he is a much better sleeper!
That's my 2 cents...do with it what you will.