It’s important to keep in mind that a “hook” is whatever offers added value to a particular college or university. The term is often used to describe broad, common categories - legacy, athlete, URM – but it also can mean different and very particularized things to particular colleges. For example, to any school with significant gender imbalance, being a member of the underrepresented gender is a hook.
When my daughter was applying to college, Bard was very actively recruiting STEM majors. They had invested money in a new science science center (which opened about 10 years ago) - an it was very obvious that they hoped to bring in plenty of STEM-oriented students to fill it. So at the time, a STEM orientation was a hook for Bard – and it still is for many LAC’s with more of an artsy reputation – but obviously not a “hook” for applicants to schools like Johns Hopkins or CMU which have very strong established reputations in the sciences.
It can be hard to figure out a “hook” for a well-rounded student who has good grades and test scores, but lacks an unusual passion or interest that stands out from other applicants. I’d assume that statistically, the majority of college applicants probably fit in that category - after all, they are high school students who most likely have simply done the things high school students are expected to do, and done it well – taken the recommended course load, including a full complement of AP’s; scoring well on standardized tests; participating in the sort of ECs and/or athletics that high schools typically offer.
But the student with the lopsided profile - like my D. or the OP’s son – offers more potential for college admission, because it is easier to figure out what their strengths are. Essentially, the goal in a college application is marketing: the candidate is making a sales pitch to a college focusing on key strengths. But to do that successfully, one need to identify the target market, which can mean going against type for that college.
The OP’s son can try to re-invent himself to fit whatever he thinks elite colleges want - which is likely to be wrong in any case, as the first impulse and the one that seems uniformly recommended is to make the applicant look more like everyone else.
Or the OP’s son can stop worrying about what brand name colleges want from him, but instead figure out what he wants for himself and what he has to offer, and then do the detective work to figure out which colleges really want what he has to offer. I think he will be happier in the long run going with the latter approach. His grades and test score are strong enough to pretty much guarantee admission to well-regarded college – it is just the top-ranked prestige schools that remain elusive.
That doesn’t mean that he completely gives up on the goal of trying to strengthen his appeal to colleges by filling in gaps. If it fits into his schedule, another year of Spanish is worth pursuing. The main point I am making is that I think it would be a mistake for him to give up on the things he really cares about – band and the science research program – because of information his mom picks up on CC about his “chances.”