Is there any way he could take this required course in summer school or at a local community college and transfer it back to HS?
I would look into this route as well. I know that our high school also offers some courses that start an hour before school so kids can fit in required courses if they want to do band/choir, etc. Some schools will also let you do an independent study to get in required courses. I would have him talk with the high school counselor and see what options he might have.
Getting a music scholarship for a non-music major is pretty rare-at least a full ride one anyway. I know many schools will give a nominal scholarship if you participate in band/orchestra but don't major in music--say $500-1000 or so.