NUMATS only takes 6th graders or higher, the only option is register directly through the testing agency and choose local high school.
Just fyi, we were told that we could request a waiver for ds10 to take the SAT through NUMATS if we wanted, because he did so well on the EXPLORE that it was pointless to take it again (I recall that your ds did as well). The only drawback is that he wouldn't qualify for the awards ceremony no matter how well he did, but I think going once was probably enough for him. To the OP - my other son took the SAT at age 11, and one of my dds at age 10 (both through a middle school gifted program) and neither considered it particularly frustrating, or felt out of place (but they were both with other middle schoolers).
This year, both my boys (10 and 12) decided not to take any standardized tests outside of school. Ds10 had a ton of gifted testing for middle school and ds12 has just moved on to other things. I think it was more useful when we were trying to get the school to realize they needed some accomodations.
As for prepping, we've never done it, and I can't imagine my boys being that interested in prepping.
I will say, though, that we recently had 3 kids (7th grade) over for a group project and all 3 had both taken an SAT prep class and were taking ongoing outside math enrichment. So, I guess it's more common than I thought (and they did do better than my son on the math section, where I think prep makes a bigger difference). I'll advise my kids to prep when it counts, and we've still got a few years.
It was really interesting, though, to do the out-of-level testing and see how they did and where their strengths are. It definitely helped guide both us and the school.
