I was so scared, just like you when DD started this. I tried every-thing, including those anti roll cusions to keep her on her back. Nothing worked, I wasn't sleeping as so stressed - so I called the SIDS people. They were excellent, talked me through some options and gave me some confidence to move to the next sleeping stage with DD. This may not be for you, but will put it up all the same.
Basically, they said that DD needs to find her own natural sleeping position(s). By forcing her onto her back at this stage, is actually putting her in more danger. They said to lay her down on her back, but just let her go. We removed all bedding, apart from a fitted matrice protector and sheet (both tight fitting). I dressed DD according to the weather and let her 'roam free'.
It took her a few nights to settle into the space and get use to rolling onto the sides of the cot. It took me a few nights also to get use to the idea and slowly, start to sleep soundly.
Anyway, thats about enough from me I think. If you are particularly stressed about it and don't get the answers you need from this post - give the SIDS people a call. Best of luck !
ps - your baby is very clever to be rolling like that !
DD is 3yr 8 months - DS is 6 months