Hi bluemagic- sorry your DS is having a tough time.
Just a few quick thoughts. Are there any other band-related activities he could try? Either a summer camp, or a small group ensemble? Our band students all get the opportunity to participate in small ensembles (quintets, trios, etc) after they get to a certain level, and it is a great way to get to know kids better and work closely as a group; it teaches and challenges different skills than playing solo or in a large group and might help him forge relationships.
Also, you mentioned academic decathlon- if he is interested but too shy to join, is there any way to get someone else to help convince him? Either another kid you or he knows, or a teacher who knows his strengths? My DD was afraid to try out for a similar academic team here, and avoided it; she finally tried out after several coaches and teachers suggested it, but only after one teacher hand delivered a note with the tryout times and location to her!). It was frustrating, as I think we all could see it would be a good fit (it was), but she is pretty stubborn. Sometimes a little push can be helpful, if you really think it would suit him. It is where my DD found her tribe, and her confidence took off.
Another idea for inviting friends might be school-related shows or activities- here, kids often get together to attend the school musicals, or each other's concerts and recitals, improv nights, talent-type shows, etc. Perhaps marching band buddies would be into watching a game or something? Do they have pep-band there? Or some other marching band alternative on the "off" season?
