At Hotchkiss they have a humanities program where all the social sciences are integrated, and the student picks an art to focus on. This curriculum is for the 1st 2 years. What it means is that they will be studying the Renaissance in history, reading Renaissance literature in English, studying Renaissance philosophers in philosophy, and playing Renaissance music, if their chosen art is music. (They can choose to focus on theater, or fine arts or photography as the other art concentrations). Add in a math, a science and a foreign language, and you are full up.
There is a little more flexibility in the upper two years, but not much. Hotchkiss is a demanding school, and a kid can only do so much. Any extra bandwidth the student has might be spent on acting in a play, singing in a chorus, competing in debate etc vs taking an extra class.
Plus, colleges want to see 4 years of English/history/science/math/language so there isn’t much wiggle room.
If languages end up being a top priority, besides the nsli-y programs several of us have recommended (my oldest did this, studying Farsi in Tajikistan), your son could always do intensive immersion courses in the summer (Middlebury, eg). Finally I believe there are German exchange programs (maybe post high school) run by the German government.
Boarding schools also can have extensive study abroad programs, including School Year Abroad which can lead to fluency.
So there are a few ways to approach the issue of foreign language instruction and boarding school.