For 9.3x74 brass, you might want to go over to Accuratereloading.com and put an ad up in the classifieds.
There are quite a few (lots actually) gentlemen there who run that cartridge and my gut says that's the place to find a 'hey neighbor' deal on brass for your rifle.
If your son really enjoys fiddling and ends up catching the double rifle bug - suggest to him that he find a pinfire double rifle in decent shape and get it running. Short of needle-fire or home-brew teat/rimfire, I can't imagine a better outlet for his cartridge/handloading itch.
I haven't had a more fulfilling handloading experience than the ground-up (tooling/brass/mould/regulation) re-animation of my 20b SxS Purdey. I highly recommend it!
Let us know when you get your rifle up and running.
Unless there's something funny about your rifle, it should be pretty easy to get a good load going for it.
I can't wait to see the photos!
Cheers
Tinker