If he is advanced, it is hard to make recommendations without knowing his aesthetic preferences, composers he admires and so on. Does he write for chamber ensembles, orchestra, solos, duos etc.? Is her aware of 20th and 21st century composers?
Again, Walden, Brevard, Boston Conservatory- Boston University Tanglewood Institute (BUTI). Bowdoin is very competitive. Atlantic Festival has an excellent program for under 18. AMF Institute: A Summer of Exceptional Music Education and Fellowship Opportunities at the Atlantic Music Festival NEC has SICPP, not sure of the age limit.
Many college age composers go to Europe in the summer. I don’t know if any take high schoolers, for example About soundSCAPE — soundSCAPE Composition and Performance Exchange (soundscapefestival.org)
Programs and application info are often posted here as well as calls for scores:
www.composerssite.com (site is down at the moment)
Walden has a thorough listing of resources, programs, opportunities here, with the added benefit of being geared more to high schoolers:
Resources: Competitions, Organizations, Partners | The Walden School