Can you clarify what the budget is, and whether you would qualify for need based financial aid? This might impact which schools would be a good choice. In deciding upon the budget you might want to keep the potential cost of law school in mind.
U.Mass Amherst and U.Mass Lowell both have good music programs. Many years ago I used to have a guitar teacher who had started out at Berklee College of Music, ran out of money, and had finished his bachelor’s degree at U.Mass Lowell. He was needing to give a lot of music lessons to pay off his Berklee debt, and would have been better off if he had started at U.Mass (either location). Of course this does depend upon a student’s budget.
McGill has a superb music program. The biggest problem I see here is that it is known for grade deflation, which is not likely to help your son’s subsequent applications to law school.
I am curious why your son does not want to consider the various SUNY schools. Again understanding this reluctance might help some of us suggest other schools that might be a better fit. Staying in-state could help to save some college funds for law school.