If you can find out the ones that do the professional ball players' surgeries - those should be some of the top ones.
Pro ball players probably get the best surgeons they can.

But as Jay mentioned, it's a gamble sometimes.

I would ask how bad are the symptoms on a normal day?
what is his avg pain level during a normal day?
what fitness level is he at?
his age?
overall posture - head forward, shoulders hunched or forward?

Has any expert PT, chiropractic, bodywork, or any other therapies been tried?

Though a clear cut case of vascular/venous TOS with a skilled/expert surgeon usually does have better outcomes most of the time.
Make sure they explain exactly the whats, whys & hows about the surgery. So you all know & understand.

Does he have a lot of swelling from the blood being backed up in his arms and not flowing back to the heart?

Drs & PTs - http://neurotalk.psychcentral.com/thread135.html
useful links & info- http://neurotalk.psychcentral.com/thread84.html