Thanks for the input. The psychologist who administered the test said essentially the same thing, that she felt the (relatively) low score in that area was likely to reflect his lack of formal education/life experience so far, and that she would expect that he would likely score higher overall should we retest in a couple of years. She showed me a couple of examples of the questions he couldn't answer, and, while I don't think they were conceptually too difficult for him, they involved some concrete knowledge of objects and experiences he simply doesn't have at this stage. I definitely see why he did poorly in that area.
