Hi Marylou,
My dd did have a couple nights where she would roll onto her tummy and not know how to roll back, and we'd have to rescue her. But, I also tried to work with her during the day on how to roll back onto her back. She eventually got the knack of rolling tummy to back, then back to tummy several times in a row, so this is no longer a problem for us luckily.
Now, she seems comfortable sleeping on her tummy sometimes, too. Hopefully that is what will happen for your ds. Sorry I wasn't much help - good luck!