I will answer your PM but just want to say that most universities and colleges include composition in their general BA (lberal arts) music curriculum, and have at least one composition teacher.
A student composer does not even have to major in music, honestly, though taking needed classes in theory and history are a good idea. The important thing is to have lessons, and performances, which can be through extracurricular organizations and especially summer programs, which are essential for networking and meeting faculty from potential grad programs.
I know several who have majored in CS or engineering or whatever and have gone on to doctorates in composition. Of course CS and music can intersect in creative work and music technology is also part of most curricula.
I am not sure what he envisions as a “career in composition.” Most composers have other jobs, often teaching, though universities are using adjuncts a lot these days.
I like what you wrote about how he can not NOT write music. That is a very important statement. That said, he may not need or want the immersive experience of a BM where 2/3-3/4 classes are music. I personally think that, long term, composers benefit from courses in art history, literature, political and social issues, and yes, of course, technology.
He can certainly apply to grad school with 3-4 good pieces and there are many ways to accomplish that. Grad students do come from all different backgrounds. I even know a couple of BA music students who went into direct doctoral programs.
It is easier to do music as an extracurricular than it is to do CS as an extracurricular. He could major in CS and minor in music, do a double major (difficult) or double degree, or just do CS and extracurricular music. Summer programs are key!