This is such a tough question to answer because no one on here is going to know your kid. His ability, potential, interest, motivation, etc. Also, it’s tough as a theoretic question, because parents of competition math kids generally only know their own kids very well plus maybe a limited number of their kids’ fellow math travelers encountered along the way. Teachers have a lot of experience with regular kids of course, but typically in the early years have very little exposure or appreciation for kids who ultimately do go on to things like USAMO winner or IMO Gold…
So, just a few thoughts from a parent of a moderately talented kid who will never be a USAMO winner but has had some more modest successes, based on our limited experiences. Any curricular math is going to be irrelevant for competition. Do it or don’t do it (my own kid never went to math class from the middle of 2nd grade through the end of 5th and I don’t think it made one bit of difference - all year end state required exams were taken with perfect scores on each), but I don’t think it matters a bit ultimately. Kids go at all different speeds. Teachers will say you need to go rigorously through classes, and not go too fast, but this is very subjective and kid dependent. What is fast for one kid is interminably slow for another, and you always need to be attuned to keeping a kid interested but not overwhelmed. As an extreme example, I remember reading somewhere about Terence Tao’s crediting his physician dad with working with him as a young kid and emphasizing the need to go slow and absorb basics. Well, he couldn’t have gone too slow as Tao got a 760 on the SATM at 9 years old, which shows some decent mastery of basic math, but competed IMO two years later, ultimately winning his first Gold at 12. That’s quite a jump in 3 years!
If we could do it all over again based on what we know now (the original reason for skipping out of math class was that it was a waste of time, as the kid was exploring algebra and more on their own at 7 years old, unbeknownst to us), I’d say do the basics through AoPS right away: Algebra A and B. Intro Geometry is also a fantastic course, much better than any you will find at any school (except maybe Stanford OHS). To be even in the game, you really need to know the material covered by the basic courses by 6th grade or so imo. Then, just take it easy and see where the kid’s interest takes him. AMC8 and AMC10 competitions are very common tests that he can start taking next year - certainly the 8. Some kids like the speed contests like MATHCOUNTS. If your son is very good in math, perhaps talk with the school into allowing him more freedom in his regular classes. Standard drill and kill homework can be a waste of time for some kids, as rules are absorbed by some much more easily than others. You’ll only know as you progress down this path.
If you post your geographic area, some parents might be able to point you to specific resources that they are familiar with. Math circles can be great fun, sometimes there are state- or county-based teams that your school might not know about or be involved in. Some parents opt for Stanford OHS courses for advanced elementary kids who are ready for more rigorous study, because absolutely no regular school is going to be super rigorous no matter what they tell you.
Last, about Exeter, its success has nothing to do with the curriculum. More than any other school, Exeter recruits specifically for the top talent. Kids are found at MOP Blue sometimes (which means <9th graders who have already done quite well on USAMO) and if necessary standard application deadlines and procedures are ignored. It’s symbiotic as well. Zumin Feng is very well known of course, and many kids look to Exeter specifically to work with him, so the school gets a hugely disproportionate share of the most promising competition kids. It’s a small world anyway, and many kids meet the adult mentors like Feng, Paul Zeiss, Sam Vandervelde, Po Shen Lo, Andreescu, etc. at summer camps, math circle talks, etc. Mine has spoken with them all and our kid is just a very modest talent in this world.
Getting back to Exeter, many “regular” kids report mixed results from their problem centered approach, with many saying it is confusing and poorly presented. It’s just anecdotal, but my feeling is that the more standard approach at Andover, say, works “better” for the majority of smart kids who are not going to be diving in headfirst into competition. And while top boarding schools of course will have many smart kids, they are looking for all sorts of other attributes in their students, so math classes at places like TJ or NCSSM or even Saratoga High School (public in CA) can allow much more intensity, as the average ability and motivation to study STEM at these schools will be higher than at boarding schools.
Most of all, just have fun! Some kids take to the competition stuff and some don’t. It doesn’t mean you will never be good at math if you are not good at competitions. Kids also shouldn’t be discouraged by failures, and most aren’t, at least those who stick with it (I guess that’s by definition, lol). How many tennis kids really think they can be the next Federer? Doesn’t mean they stop trying. But, like athletics, math competitions are all out in the open, so it takes a certain type of young person to deal with the inevitable failures.
Anyway, hope that helps. OP, feel free to PM with your geographic location and school and if I know anything specific that I can offer I will!