" So, if my son is not good enough then he will have no choice but to drop out of music altogether as he won’t find a good teacher to coach him. "
the way around that is to have him include in his list of colleges those that have great music programs.
So, once again, have him visit colleges that have strong music programs [ not just those with conservatories] AND require auditions in order to take lessons from the music profs, before their audition deadlines.
that way he can have his cake and eat it too-
I do know the structure of colleges with strong music programs, as that was a big must have for DS when he was constructing his list of colleges to apply to.