I would suggest he read “The Rest is Noise” by Alex Ross and listen to a lot of 20th-21st century composers- agreeing with @DrPrimo.
However, for a beginning composer, I think Walden is best because all levels attend: Walden looks for creative kids, and there are advanced high school composers as well as novices: all seem to write amazing pieces. Boston Conservatory and Brevard are also good.
At the high school level, I don’t think you have to worry yet about the particular aesthetic of teachers, at least until you have established a “voice” of your own or a direction. A good teacher is not directive but facilitates the growth that you have on your own.
To apply for a BM you need a few pieces and it helps a lot to have them played by excellent musicians. This is one of the main values of summer programs. Occasionally you can find local/regional musicians who will do a reading for free or low cost. For BA programs it helps to have at least one good piece, to include in a supplement.