From a max number/access standpoint, it makes sense, of course, to select the instrument which is least affected by his relative weaknesses. From a clinical interpretation standpoint, however, it may well be more informative to administer an instrument which allows you to compare his motor-involved and motor-reduced performance, such as the WISC-V or KABC-II.
As the parent of a 2e ds with fine motor challenges, unless you have some reason to need to get the highest ability score possible, I'd recommend going with a test such as the WISC that includes a few subtests that rely on fine motor output (timed and untimed). The thing that's been ultimately the most valuable for us out of ability testing wasn't the actual total "IQ number", but instead seeing the spread in subtest scores, relating discrepancies back to where we saw our ds struggling, and using that knowledge for further testing and eventually a plan for accommodations that ultimately allowed ds to be able to work in academics to his full intellectual ability. We had a few struggles in elementary school explaining that the FSIQ (WISC) wasn't valid and what the GAI (WISC) represented, but awareness of gaps in abilities, as well as the concept of 2e, seems to be spreading. As long as *you* understand what's going on with your child, you can advocate successfully using "spikey" data.
In addition, he struggles with a fine motor delay and a rather extreme tendency for perfectionism so timed writing or subtests like block design pose a significant challenge.
FWIW, until our ds was diagnosed with DCD and dysgraphia, we thought he was an extreme perfectionist because of the way he approached his academic work. We (parents) were *soooo* wrong about that! It's not always easy to tease apart what's up with a child who's 8 years old - but having a test that was able to illustrate the difference in ability scores when using fine motor tasks vs not, timed vs not etc, helped point his neuropsych in a direction for a small bit of further testing, which led to an accurate diagnosis, which led to accommodations and... perfectionism really wasn't the issue at all. If you see perfectionism in other areas of life that don't call on academic and fine motor skills, then it probably is really perfectionism... but if you're seeing it when he's tasked with a fine motor challenge or in other academic situations, it's possible what looks like perfectionism is related to some other type of challenge.
Best wishes,
polarbear
