It must be a tough call because I remember Maddy started rolling at 4 months and once she could roll over she would go to sleep on her tummy no matter how much we tried to get her to sleep on her back.

Have you tried Cooper in a sleeping bag? It won't fix the arms waking him up issue, but perhaps it'll make him feel more secure without being completely binding? I can certainly understand you're concern, if he's lying on his belly and puts his head face down to rest, I too would be concerned about him not turning his head especially when his arms are restrained as such.