One thing to keep in mind about those subscores is that they're based on a relatively small number of questions. I've got an old ACT here (from a practice book) and according to their scoring table it is only one or two questions per point for each subscore in math. So especially with a time limit, any pattern of skipping could affect one subscore more than another... like if he skipped anything with a graph, that could bring his score way down just in that area.
What I've also noticed with a kid a tutor is that the balance of scores does seem to reflect what we prep with... so when we were focusing on trig, his Plane Geometry/Trig score shot up (again on old practice tests) and the others didn't... so reviewing just the sorts of questions he missed might get a more even score if you retest.
We had a similar situation with DS(9) and the Explore... between his testing at 7 and his testing at 8 his science score just sat there. It was fantastic in either case, but it didn't move! He'll be taking it again this year and the suspense is killing me! LOL But I figure of all the concerns to have, worrying about the difference between great and super-wonderful isn't too bad
Of course if we were on the edge of admission to a program like TS it might be more stressful, but it sounds like he's almost in range there and one more try might do it.
