My dd11 is currently a 7th grader and took the SAT this past January. While she did well, she didn't do well enough to be in the SET category, but she still has next year if that's something she wants to shoot for. We didn't do any prep for the SAT.
I'd be inclined to let him chase his brother more than to afterschool him in math in order to have him do well on a test five years down the line unless he was the one actively seeking extra math.
Would it be possible to just keep up whatever minimal enrichment you are providing now and attempt to get him subject accelerated in math at school so he is getting appropriate curriculum there moreso than inappropriate math during the day supplemented with more work at home? Maybe the school would let him do something like EPGY at school in lieu of some of their curriculum? If he got through a good chunk of their high school math, I would imagine that he'd be as well prepared as most high school students.
I do know that I've heard that some talent search kids prepare through those test prep companies -- Kaplan, Princeton Review, etc. That would probably give him test taking strategies which would further boost his chances of a high score, but if you're going to go that route, I'd wait until 6th or 7th grade I guess.
