Sorry I don't know about any in the St. Louis area (I'm from KC), but Badger Cycles is in Lawrence, KS.
Rob would probably let your son participate, but you'd probably have to come out and stay a few days in Lawrence to get it done. Rob specializes in both lugged and brazed frames, and also has a tig machine.
As Walt's told you, Diablo is gone. Not much to miss there, in my opinion. Rob (Badger) is about 100x more of a bike builder than Mike at Diablo ever was.
Good luck in your search.