I was a music ed major years ago. We only had 2 tracks, vocal and instrumental (which had to learn wind, strings and percussion). Depending upon the school there may be 2 or 3 tracks.
There is no need to learn a wind instrument at this point. Focus on your primary instrument(s). Viola tends to be a little less common and can have more opportunities for scholarships, depending upon the school. But if his interest is band there is nothing wrong with percussion (although I might lose my brass card saying that 
). The audition is the most important thing, prepare for that.