For leisure reading, his books appear to be on 4th-5th grade level. I looked it up.
What grade is he in at school? One thing you might be up against is that there are other children in class or in his grade reading at the same level, so in reading he doesn't stand out as hugely intellectually gifted, even though he's one of the top level readers. The thing about reading skills in early elementary is that they really are all over the place among a typical classroom and grade.
In any event, don't let them intimidate you by saying he's "average" in anything - learn all you can about what they are using to make that statement, and ask questions - ask *why* the test shows he's average, ask what was tested etc.
Best wishes,
polarbear
