My best advice would be to discuss it with the folks (not the newbies) who ride track at the VD in Seattle. Surely the old-hands have the info you seek and they might know of a framebuilder who's done track frames.

Richard Sachs though his waiting list is long; he might recommend someone with a shorter list. Bill Davidson (Seattle I think); he's been around the block. Mark Bulgier (also Seattle). Not sure if he builds commercially but he's another one who could direct you.

Aside from that I'm guessing that your son is a track beginner. I think a good used bike would be the way to start. Once he's settled into track riding and his position/likes/dislikes have matured a little...then a custom track frame would be most likely to be what he actually wants/needs/benefits from.