I’m sorry no one has answered this. I think it is a busy time for everyone. Your son is a good example of someone who is a great candidate for a music ed program- because he wants to teach. Many seem to think of the music ed route as a backup, so to speak, so it is great to read about a young man who really wants to be a band director. My only suggestion is to make sure he know that sometimes these things change, and he can decide how much flexibility he wants at this stage of the game.
Is he at all interested in music performance, or a general music major? Meaning BM or BA? Music ed is a great path so just making sure 
Here is a good essay to read about music studies and choices:
http://www.peabody.jhu.edu/conservatory/admissions/tips/doubledegree.html
My first reaction is to say, advise him to do what he really wants to do, without gearing his choice to admissions. This is because doing what he really wants to do and loves will lead to the right choices in the hard decisions that lie ahead.
That said, if he is really interested in music ed, perhaps he could talk with someone in a music ed department or admissions to get an answer he feels really secure about.
How much would a new instrument take away from his string and percussion work? Is there any chance he will want to focus on one of those in a performance degree rather than music ed? At the undergrad level, some do that and then do music ed in a grad program. In fact, I know someone with a BA in music at a liberal arts school who then went back and got a master’s in music ed. He runs the music dept. at our school.
There are many paths. Just want to say that things are flexible, more than it might seem to a junior in high school. And a lot can change in two years.
If he wants to explore a wind instrument and would enjoy it, and it doesn’t take away from other interests, fine. If he wants to deepen his experience with string and/or percussion (with or without orchestra) that’s fine too. If he is interested in performance, one thing a lot of us learn is to go outside of the school environment, get a really good private teacher and also look at summer programs.
Lots of possibilities. It isn’t a simple question unless he is really really sure he wants to do music ed. And is unlikely to change. In that case, staying in band or orchestra continues to give him a role model I suppose, but trying a wind instrument offers diversity in playing. Again, if no one more knowledgeable comes on to answer (I know little about music ed) then I would have him call or email a music ed department to see which choice is more helpful once he is in a program. ( I think that is a better way to put it than which will get him in, and it may not make any difference.)
Good luck!