Specific familiarity with the types of math questions which appear on the SAT is likely to be the only strategy that really works with such a student-- and honestly, I don't really consider this "prepping" in the traditional sense, since your DS (like most of the kids here) is completely capable of a perfect score-- under ideal conditions.
So you're trying to tweak the match between your DS and the conditions of the test. Nothing wrong with that.
Practice tests under timed conditions which mimic test site conditions is one method-- let him figure out what "pace" is best. DD found that there was a sweet spot-- if she worked slower, her error rate increased on the lower level items, and if she worked any faster, she made errors on the harder items.
She used extra time to run back through the section and rework problems-- which also helped her catch errors.
I have to say, though, that the SAT math sections were just not favorable for showing what she can do. She's not an algorithm type of mathy kid, and that's really what it tends to take to earn 750+ on it.
The ACT was much better for "thinker/tinkerer" kids like my DD.
