Girls help... Suggest something because I don't know what to do anymo…

Girls help... Suggest something because I don't know what to do anymore. The child's head becomes flat from behind... I put him on his side and put a small pillow between his legs so he doesn't roll over on his stomach it's uncomfortable he cries, he can't sleep like that. I lay him on his back and turn his head to the side, I leave, after 5 minutes he turns it straight up, puts it down... What should I do????