I have considered music ed many times in the past few years thinking about college, and while I was more focused on vocal ed rather than instrumental ed, it is a good idea to have a few instruments down. My vocal teacher had mentioned to learn either piano or guitar for me in addition to my voice. As for instrumental ed, I would say that strings and percussion is a good mix. Also, if you want more advice, you should have your son contact his orchestra director and ask him about his experience going into college as a music ed/music major.