I would definitely send an email to the principal and to the guidance counselor or maybe to his favorite teacher (depending on whom I felt most comfortable with, and depending on which person "gets" my kid the most). I'd say, just in an upbeat way, "Hi! Just wanted to let you know that DS did xyz, and only __ kids in the whole [district, state, etc] did this! This really reflects well on our school, and we are proud of DS."
I'd do this first, on the chance that the school just didn't realize/take notice. Especially if the school (like our former school!) is weak in music, they just don't "get"/take notice/stay on top of music achievements. My kid's elementary/middle school was so weak in music that (out of a population of 60 per grade) they had only THREE kids in the orchestra by 5th grade, 1 by 6th grade, and none by 8th grade.
