I wonder if age is a part of his reaction to those 4 days of relative inactivity. For a 15 year-old, did they promise a very structured program beginning on the first day?
I also wonder if this is more of a foundational program than he wants. In many fields, even the most talented musicians can benefit from going back to the basics a bit.
We know percussionists in DMA programs who were talented at 15 but have studied for 12+ more years than your son.
There may be disappointment for a young musician who is clearly talented and used performance as a priority. Does he do lessons every week at home? What kind of training does he have?
I honestly hate to see a great school deliver so poorly for a summer program, if that is indeed what is happening, and hope he can stick it out and get some benefit out of the program despite whatever level of disorganization is present for such a large program.
Also there are issues particular to percussion that clearly need to be resolved. Access to instruments, access to practice opportunities, are easier for other musicians.