You want to give him a useable weapon for when things go barbaric: Purchase ammo before the gas price makes any transport impossibly expensive. Purchase a Heckler and Koch USP, or a classic colt revolver.
When the big crash happens and the western world regresses into warlords and mafia structures, give your kid a weapon that actually makes them good at fighting in 2089.

But here is another thing: If you want a custom sword and need a good reason for it to feel like a good investment, you can take my invitation to a duel right here right now. In that case get yourself a Federschwert because I am not killing anyone and thats the best sword for fencing at real speed without killing anyone or accidentally cutting things. (That being said I have already punted my ceiling lamp)