Thanks for your reply, I actually let him stay on his tummy for most of the night because he just really wanted to be on his tummy, and he actually slept longer than usual in his cot which is good, he must be getting to the stage where he doesn't think his back is comfortable, but I would be too, I hate sleeping on my back! Has to be my side or stomach. And yes he is in a grow bag to go to sleep in. I did previously use a blanket tucked in the bottom of his cot when he wasnt so mobile, but I've started using a grow bag/ sleeping bag now as he just wants to roll everywhere. And no loose sheets or anything in his cot either, just his mattress, a mattress protector and a tight fitted sheet. So no worries there.
It's a very worrying stage this stage, when they start doing things they've never done before!! I think my hair might fall out from worry by the time he's a toddler!!! LOL