DS9 isn't there yet, but it's foreseeable that he will be. In standard-US terms, he has most of AP Calculus and most of AP Statistics still to go. In UK terms we have a bit more flexibility, because there are more options in the final two years of school maths than any one student normally takes; if we have him do it all (and whether this is sensible is one of the questions in my mind), it'll keep him going for a few more years.

Here's one fairly typical syllabus document. *

I had a quick look at that

157 page PDF document. Obviously your son should just do the whole lot if possible. From your comments in various threads I wasn't quite sure how he's covering this material, since he's just going to his regular grade in a B&M school. How is he doing it?

Yes "Mechanics" is part of Physics in the USA. Also I see the subject area called "Decision Mathematics" which looks more like Discrete Mathematics. That's an area (if interested) that he could go a lot further in without clashing too much with the university courses (since the area is somewhat neglected in many departments).

The UK K-12 syllabus certainly covers more than in the USA. I assume that's due to earlier specialization, and due to not lowering the level so that more people can reach it. It's true that one can do 100% maths in a UK undergraduate degree, right? American undergraduate degrees are far too broad, meaning not enough maths gets covered. Anyway, that's an argument for covering material early, just to get to a reasonable level.

Someone was questioning in another thread, why would anyone bother to get a PhD, just to end up teaching high school math. Well the answer is that you need a PhD to get a job at a university.