I've never flown with oxygen, so hopefully some other experienced people will comment on that. My guess would be that they'll err on the side of ignoring you, rather than hovering.

As your son is four and not an infant, I assume the diapers and other equipment are medical needs, in which case they should not count against your carry on allowance. I'm very surprised that his portable oxygen concentrator would count as hand luggage - if I were you I'd call them and double check that.

Welcome to FlyerTalk, and enjoy your trip.