In general, a waitlist is a rejection but a hedge for the school should they need more bodies.
That said, they typically would not waitlist a student that they don’t see as a fit.
Too many programs - especially in a major like music - have a lot of fantastic applicants but not enough spots.
You should embrace the schools where you’ve been accepted and plan to go to one of those.
In most cases, wait list decisions come after May 1.
Some schools take no one or few off the waitlist. Other schools take many.
Some schools - like Case Western, wait list 30% of applicants.
In the last Common Data Set, they say they do not wait list. So I wonder if this is specific to the School of Music. Certainly a concise Letter of Continuing interest letting them know why you’re a good fit and you’ll absolutely attend if accepted is a smart idea.
Best of luck to your son and I’m sure he’ll find a great place somewhere.