Welcome!
The simplest response to your WM/PS (or, together, CPI) question, honestly, is not to worry about it. None of the measures of WM or PS are remotely below average, and most of them are above. These two indices are more mechanical in nature than the other three indices, which comprise the reasoning-weighted General Ability Index. Not uncommonly, individuals, especially young children, will have motor speed or attention skills that more closely resemble those of their chronological age-peers, even when their higher-level reasoning and conceptual skills far surpass them. IOW, his handwriting may be not much better than the next child his age, even though he can read and comprehend many years ahead. Your concern about perfectionism may or may not be reflected in these scores, but since you have independently observed it outside of testing, you hardly need testing data to encourage addressing it.
I am, of course, simplifying a bit, but the bottom line is that there are many reasons other than ADHD or other disabilities for the CPI tasks to be lower than the GAI tasks, especially in a very high cognitive individual. Most of them are quite innocuous, and have more to do with asynchrony than anything else. Perfectionism is one of the less innocuous explanations, with the most likely effects falling in the PSI. (That may have something to do with the SS score, which is his lowest subtest. Or the fact that it is the last subtest administered, and may have been affected by fatigue.)
As to how they are affecting him: the typical scenario is that conceptual understanding far outstrips paper production. In a large class setting, where teachers are heavily reliant on paper (or digital proxy) products for assessing student skills, it can be very easy to inadvertently underestimate skills, by mistaking classroom attainment (what he produces for the teacher in structured assignments) for academic achievement (his actual range of academic skills, when assessed in a controlled environment, on properly validated instruments). The former is easily affected/depressed by factors such as handwriting skill or speed, poor fit in instructional level (which can affect motivation and attention), and social-emotional factors (anxiety, perfectionism, perceptions of the teacher's supportiveness, approval, expectations, etc.).
