Like Cricket, I also wouldn't be concerned with the WISC/WIAT test results, as they seem to line up pretty well with each other, and the lows on the WISC are most likely due to your ds' hypotonia. The one thing I might wonder about is vision, only from your description of your ds looking at the blocks in block design from an odd angle. My gut feeling at first was that hypotonia prevented him from doing well on that test (in block design a child has to manipulate blocks to replicate a design, and it's a timed test)... but getting up and looking at it from an odd perspective *might* have something to do with vision.
On achievement tests (WIAT) any subtest that is labelled "fluency" is usually timed, so again your ds' hypotonia most likely impacted his academic writing fluency subtest.
The GAI is calculated from PRI + VIQ - working memory and processing speed subtests are not included. The PRI and VIQ subtest scores are all very close in range (and very strong). I am not an expert, but I don't see anything that would impact the GAI. His FSIQ will be lower due to the WM and PSI scores, but most gifted programs we've looked into accept GAI in place of FSIQ.
As Cricket said, you'll most likely have to advocate at school to be sure your ds isn't held back due to the challenges of his hypotonia and possible OCD. While his WISC scores aren't stratospherically high, they are VERY strong and having the weaker scores in WM and PSI is *very* much expected due to hypotonia, so really I see what you have is strong - it shows strong ability, the scores are consistent and they make sense. Being able to show that scores make sense and support what you see in your child helps a lot when advocating.
Best wishes,
polarbear
