My DS1 used to do the same. I would put him in on his back and he would roll to his stomach. I was really worried about it but there was not much I could do unless I sat there all night and watched him. Sorry I have no solutions on how to stop him from doing it but just wanted to say that DS did it from about 6 months and he was fine. We had absolutely nothing else in the cot with him. No pillow etc. I think now it is starting to warm up it is safer as well because they don't have the blankets. Maybe someone has some solutions. I know some people use the anti-roll pillows on little bubs but I'm not sure about 7 months.