I know a lot of parents curate their kids' school careers with an eye to college apps starting much earlier than this. I guess I've just been of the mindset to let them be their best, most authentic selves and let the right college fit happen. Now I'm second guessing that. High school parents, especially BTDT, what have you done and do you have any regrets?
I agree with your "let them be their best" approach.
A hard part of raising kids is letting them make their own decisions. It's hard in part because you get so used to HAVING to make the decisions when they're little. As you know, high school student is capable of making decisions, though they generally need more older-person input than a college student or a 28-year-old.
FWIW, the review of algebra 2 concepts is important, especially if algebra 2 is done correctly. There's a lot of material there, and going over it again is important for kids. I say this as someone who knows people who surpassed the DYS requirements substantially and still needed that review. So, how does your son know the material covered to date? Are they still reviewing the stuff from algebra 1 or the easy algebra 2 stuff? Important: how familiar is he with the rest of the course?
My advice is this: first, ask your son to see what the precalc class covers. Does it skip stuff he'd do in algebra 2 and hasn't done? Does it barrel into calculus (a trend in some places)? What's the pacing like? What about the teacher and textbook? Is the class algorithm-driven?
Then present pros and cons as you see them, and let him decide. As you know, mathematics education these days tends to focus on algorithms over understanding. If he stays in algebra 2 and gets easy As, would he be interested in spending time gaining a deeper understanding of the material from another source? That kind of thing.
Ultimately, our children will have to chart their own paths, and IMO, the best we can do for them is teach them how to make an informed decision when they're young, as a way of preparing them do so on their own when we're not around.