A couple thoughts and questions:
* Do you have any ability testing to go along with achievement testing? If his ability is very high and the achievement isn't quite there yet, I'd be less concerned about pushing to get the achievement up.
* If you are going to push for one area, math does seem to make the most sense of the two and I'd probably just focus on that one area for now rather than tutoring in both math and reading. Is the level of reading required for 3rd grade math in terms of word problems, directions, etc. just too much with his current reading level? If so, I might back off on 3rd grade math for now and try something else like asking that he work more independently on 2nd grade math or do something like EPGY independently for math until the reading catches up.
* In terms of whether two years of acceleration is asking too much, I'd say that it may not be but it totally depends on the kid. I've known kids who are not gifted but who are fairly bright and who work hard who do fine with one year of acceleration. I also have one child who is HG but for whom math is her weakest area (probably MG in math) and she's done fine with what amounts to 1-2 years of math acceleration. She was very young for grade, skipped a grade, and is in the accelerated math program in her grade.
Point being, brilliance is not required to be a good candidate for a year or two's acceleration in one subject.
