It sounds like you need some further info before you'll really be able to know "where to go from here". I also expect you'll need testing beyond ability + achievement, but fwiw, I think you may learn something from the WJ-III if the tester administers both the ability and achievement tests. I have 2 2e kiddos, and when they were given the WJ-III Cog there were more subtests than on the WISC, and there was a lot more information contained in their reports due to the way the test results (ability + achievement) could be grouped together in different combinations to assess different types of abilities. Long story short, it made it much clearer what their specific challenges were. We still needed further testing, but I really appreciated having the details from the WJ-III.
For just right now, I'd wait until you hear what the tester has to say after the WJ-III tests - she may have further testing she'll run based on those results - and she may not have suggested other testing yet since she feels she can't trust the results of the WISC. I'd also ask if any of the lower score subtests your ds had on the WISC are tests that rely on vision - since he says reading gives him a headache, I'd just wonder about vision.
It might also help to know - does your ds only have behavior issues at school, or is he having similar issues at home? Is homework frustrating? Does he have challenges in other group settings, with other kids?
Also what type of professional is doing the testing? Is it part of a neuropsych workup or just an educational eval? Was there any kind of parent interview before testing asking questions about developmental history, behavior etc? Any plans for a functional behavior analysis?
polarbear
