Nice looking goat, one to be proud of, I hope he is, sounds like hes got the right kind of Dad, mine are a bit younger but showing the right signs so hope I can emulate you in the next few years.
NC would be Nitro Cellulose as in who knows quite what they used but I'd think that the info is out there if only you know where to look. I would go with the previous posts & work up a load that you know is safe & that gives you good accuracy. Share the chore with your youngster, its all good experience & part of the the big piccy. Proof loads give no indication of accuracy, just that the rifle didn't grenade at that load. A proof load is normally a fair bit higher than what we would think of as a working load so don't get hung up on it.
Steve.