I’m not sure about European pre-college composition programs.
In the US, Juilliard started a 2-week summer composition program this year.
compmom already mentioned AMF. Their website seems to be partly broken right now but check back later, definitely a good option to consider.
Bowdoin only accepts 8 composers and most are age 18+, but they don’t have a strict age limit so it’s a possibility. A few others that are also highly selective:
NYO USA accepts 2 composers each summer; the program is full scholarship. Their website doesn’t give much detail about the composition program but there’s a Q&A here with more info.
Yellow Barn Young Artists is open to ages 13-20, but it starts in early June so it might not work schedule-wise, depending on your son’s school schedule.