I think you and your son might benefit from reading the Double Degree Dilemma essay posted closer to the top of this forum. It discusses different ways to study music (not just double degree) and helps clarify priorities.
Many of the state universities and some state colleges have BM programs, which are performance degrees. In a BM degree, 2/3-3/4 of classes will be in music. These same schools may offer a BA in music, which may or may not have some performance component, and offers 1/4-1/3 classes in music. A BA is often an academic music degree with classes in theory, composition, music theory, technology, ethnomusicology and so on (BM’s will have these too of course but much more applied music).
It may also be possible to major in something else entirely (Math?) and do private lessons and perform in extracurriculars. There is some intersection of computer science and music these days, if that interests.
One thing to watch out for is that when there is a BM program on campus (conservatory, school of music), the best teachers and opportunities may go to those students. This may be fine. And there are exceptions. But check it out.
It sounds like your son does quite well in high school and it’s wonderful that he wants to continue music in some way. Kids like this can often apply to different schools that represent several different options and decide later in senior year.
BM’s and double degrees usually require prescreening and auditions. BA’s usually don’t: but a music supplement can be sent. Colleges with BA programs often have auditions in the fall of freshman year for extracurricular performance.
Double degrees may be available at state schools, and there are also smaller schools like Oberlin, Bard, Lawrence, Ithaca, and others that offer double degrees with their colleges and conservatories. And schools with more flexible curriculums and fewer gen eds can make a double major a little easier.
Can he join a youth orchestra this year? Or apply to summer programs for next summer? This might help clarify and give him some idea whether he is competitive and he would hear about various schools…and meet teachers and peers.
ps In our experience Purchase is a great music school but very competitive