If you don't have a bean bag, a whole mess of folded up blankets will work too. Or a boppy covered with a blanket.
Keep the room where you plan to shoot extra warm. Bring a cheap space heater if you can.
Try and plan it so that you arrive right when he's normally ready to be fed. Then mom can feed him while you're setting up, and hopefully once you're ready he'll have a full belly and will be ready to sleep.
