In our experience with some selective schools (Harvard for instance) the music supplement was very different from the kind of portfolio submitted to conservatories. My kid actually called admissions and asked about cuing the one composition (note that it was one piece) being submitted and they wanted them to cue “the best three minutes.” No audition. Once on campus there are auditions for ensembles, orchestra and so on. Composers don’t need to play an instrument, generally.
Interviews are often just the regular interview that all applicants have, though you can request to meet with the music department (my kid did this at Yale).
The supplement would include a music resume and letters of recommendation (1-2) related to music. These tend to play more of a role for BA programs than for BM’s.
Your kid is only a junior so things may change. It might be helpful to know there are many different paths with these two interests. I know someone who did science at MIT and went on to a doctorate in composition, and someone else who did science at Bard and also went on to a doctorate in composition. I also know a musician/composer who is doing very well and did not major in music at all at the undergrad level, then got a doctorate in something else (akin to musicology). One other undergrad musician went on to a doctorate in science. And so on.
Composition can be done “on the side” as an undergrad, as a minor or a major, double degree or double major. Combining music and science can pose some scheduling issues but a composer doesn’t necessarily have to perform so that frees up time. Find out what is available to non-majors: are composition forums only for majors? Are lessons available with a good mentor, funded, for credit? A composer could take electives in composition and theory and focus on the science degree, but would have to make sure those electives are not just for non-majors.
Another thing to look for when having two intensive majors is the distribution requirements. For instance, Yale has more than Harvard (or it did, I am some years out). Is there a creative honors thesis in senior year (Harvard has this thanks to John Adams)?
A junior should continue lessons, learn theory, preferably at a conservatory prep, and pursue summer programs. Of course there may be science summer programs and science projects during the year so keeping things sane is important! For classical composition the challenge for many is getting pieces played by excellent musicians, which is still preferable, though a MIDI may be acceptable- ask about that. Many are working on DAWS (digital audio workstations) these days.
He is young but does he have career goals in mind or is he just pursuing what he loves regardless of outcome?
ps I know he is only a junior but some of the questions about programs may help guide his activities now….Has he read the Double Degree Dilemma essay in the Read me section here on the music forum, pinned at the top?