You also mention his musical skill. Young adolescents begin to develop strong areas of interest about this time. Your son, who wants to be popular, has realized that musical skills are fun and also valued by peers.
I would recommend you first determine why he received the grades he did by checking with his teachers. If the reasons are study skills or disorganization, he may need guidance from you on how to improve in these areas. The middle grades are also a time when kids become more vocal in criticizing each other. I wonder if your son's classmates criticize A-students as being "brainiacs" (or some other negative term), and he wants to avoid this label.
It's important to acknowledge that peer acceptance can be a real concern for him, but equally important to remind him of what your family expectations are for school performance. Actually, a lot of high-achieving students can be found in band and orchestra, and you may want to encourage your son in keeping those friendships.
I would like to recommend the following three books to you:
1. Why Bright Kids Get Poor Grades, and What You Can Do To Help by Dr. Sylvia Rimm
2. The Roller-Coaster Years by C. Giannetti & M. Sagarese
3. Too Old For This, Too Young For That! (a middle-school survival guide for your son) by Dr. S. Mosatche & K. Unger.