We had DD prep using DIY methods, knowing what I know about the psychology of high-stakes testing and anxiety-vs-performance neuroscience, and knowing her peculiar quirks very well.
I bought two different practice test booklets-- one with 6 or 7 practice tests in, and another with I think 4 of them.
Then, I had DD take a run at the whole shebang-- timed, exam-like conditions, started at 8 on a Saturday morning, and we saw what patterns existed, if any.
Her math sections were slow, and she wasn't very accurate. That is, she didn't finish with any extra time, and made a lot of careless errors.
So then I had her read the test prep tips in the book I had (think it was a Barron's book), and she ignored those sections where her performance was already 98th percentile or higher, since "tips" were likely to mess with her Zen in those areas. We only wanted to tweak what WASN'T workable. Not what WAS.
This was the point at which we opted to not have her take a prep class-- because those are one-size-fits-most, by and large, and that was not what DD needed.
So she tried a couple of tweaks each time she did a practice test-- and saw what worked and what didn't, and made adjustments accordingly.
It was a relatively time-intensive thing, but not especially high-stress, even so. Maybe 16 hours total investment-- she didn't do "full" practices most weekends-- only the math sections or the essay, where we knew that she needed to optimize her approach.
I'd have him try a practice test under completely timed, test-like conditions, and see how much he really NEEDS to invest in getting things perfected. It might be that at that point, you both decide that his performance is likely to be "good enough" and leave well enough alone aside from running through another test a couple of weeks out, just to keep the pacing he needs fresh in his mind.
Doing test-like conditions and evaluating how they go is also a way to add time-and-a-half and see if it makes a BIG difference in his scoring. It might not-- and if it doesn't, then it won't be worth the trouble and headaches of getting the accommodation if it isn't something that will bump his scores.
