I got a perfect GRE score (on all 3 parts), which back then were math, logic puzzle stuff, and english/analogies/whatever the hell that stuff is called. ELA?
Anyway, great, that and $10 will get me a latte.
Then I started getting solicitations to work for Kaplan and tutor students for the test. They offered (wait for it)... $8 an hour (this was in 1999, but still). And you had to do a ~30 hour training course that was unpaid.
I wondered who on earth would be smart enough to do really well on their test and also dumb enough to take the job.
IMO test prep is pointless unless your kid struggles with standardized tests more than their general ability would suggest. There are a few folks out there who do benefit. But OP's kid is probably just a smart kid (or at least good at not making dumb mistakes on easy material, but that's another topic), full stop.
-W