You didn't mention which shoulder will be operated upon -- and it makes a difference regarding times to his returning to music. If it's his left shoulder, it will not have to 'abduct,' or lift out sideways from the body, as much as would the right side. Thus, surgery on the left side might allow an earlier return to playing, if biceps/brachialis function during playing is not a postoperative issue with the surgeon (obviously, you'll have to pose this question specifically to the surgeon).
From my experience as both an orthopaedic surgeon and a performing/teaching bassoonist, it depends on what type of repair is done to the glenoid labrum (or lip -- this structure is part of the joint that helps keep the humeral head seated in its socket, the glenoid, and not part of the muscular rotator cuff). Much of the restriction on playing comes down to how effectively your son can play without using unnecessary muscles (including those that may have been cut or split in order to enter the shoulder for the labrum repair). Obviously, arthroscopic repairs preserve the muscles better than open operations.
Even though your surgeon is not familiar with a bassoonist's needs, perhaps some of the questions I've posed can be useful when you ask him or her about the specifics of playing postioeratively. If there are questions from the surgeon, please request that he or she contact me for discussion.
Dr. Bill Dawson, bassoonist and teacher
IDRS medical consultant
Past President, Performing Arts Medicine Assoc.
Author of "Fit as a Fiddle: The Musician's Guide to Playing Healthy"