Side sleeping

Me

My 10 week old baby boy will not stay on his back when I lay him down in his pack n play bassinet. He rolls over to his side and wants to sleep like that. I’ve just tried to roll him back over to his back like 10 times and he keeps rolling onto his side. I know back sleeping is best.. what can I do? I feel like I can’t sleep because I’m worried about him rolling onto his belly. Thanks!!