If he's already at the top of the test (which was likely the case in year one, when it was the 2-5 test), then negligible change, or even a slight decrease, is not surprising. A slight decrease on changing to the new level is also not strange. He's still near the top of the norms on the 6+, so there's not much space to move up.
Math is different from reading in the respect that exposure to more challenging skills and concepts is generally controlled by grade-level curriculum, while reading comprehension is mostly more of the same once you pass the fourth grade reading level, and consequently more a function of practice and native cognition than instruction. So he doesn't need to be taught reading to fly ahead (and top out the sensitivity/range of the test), but does need to be taught at least some math.
