To the best of my knowledge, a top revision surgeon is Dr. Regan in Beverly Hills. Revision is difficult and he has quite a bit of experience which is what you want.
You might want to contact Mark (
GPN Artificial Disc Replacement ADR) if you can't get an appointment.
Even if finances prohibit your son from having surgery with Dr. Regan, at least you'll have his recommendation and can proceed from there.
And welcome to the forum-
Dale