I am sure you noticed the significant discrepancy between your DS' working memory (WMI) and the other WISC IV index scores(VCI, PRI, PSI). This low WMI could very well be the source of the discrepancy between your DS' FSIQ and WIAT scores. According to a recent article in Child Development, low WMI predicts lower than expected achievement in reading and math and is a significant predictor of school underachievement. Here is a link to an overview of the article:

http://www.psychologytoday.com/blog...ing-memory-problems-find-out-why-matters

Now as to what, neurologically, underlies the low WMI score, I have no idea. Your DS' WISC scores are superior to very superior except WMI.

Also, my DD had the same language in her report: her "unique set of thinking and reasoning abilities make her overall intellectual functioning difficult to summarize . . ."etc. I think that must be stock WISC report-writing code for a large discrepancy between index scores. Your DS has a 49 point spread between VCI and WMI-over three standard deviations. My DD had a similar spread between VCI and PSI.

Hope this helps some.