While it sounds like a part of the day was terrible, it is good that he did enjoy the actual playing part. As far as bullying, most school administrators take that very seriously now days. It sounds like there was not adequate supervision. While I know that kids will typically be kids, most educators are going to supervise such that severe problems don't occur -or are at least noted and stopped.
You might want to thank the band director for letting your son participate, but also explain the negative experience. If it happened to your son, it is happening to others.
While folks my age tend to think of the gueling situations most of us had to endure when we were in school as "part of growing up," there really is no excuse for it. In fact, the legal ramifactions are enormous. In fact, as an administrator, there would be a quick conversation if it happened to one of my staff that there wouldn't be a camp if we couldn't provide adequate supervion and ensure safety. That said, bands are often their own culture and administrators may not know adequate supervison is not occurring.
Hopefully, future years will be better.