Of course, I don't have the clinical observational data and any other information that the administering psych has, but based purely on these scores, I might have come to different conclusions. It may be that the poor writing performance (both in the classroom and on formal testing) is due to ADHD, but I certainly would not have dismissed that wide a disparity between test scores in the same achievement domain as insignificant, without additional data. It is not always true that dysgraphics cannot spell in isolation. Spelling (and often handwriting) are higher-level cognitive tasks for them, rather than automatic ones, like they are for NT learners. In isolation, some--especially the very bright--are able to solve spelling problems at a high level of accuracy. This does not mean that they can apply mechanical spelling skills at the same level of accuracy while simultaneously completing other higher-level cognitive tasks, such as composing meaningful and structurally sound language. Nor can they devote as much cognition to the language when they are attempting simultaneously to maintain accuracy in mechanics. Likewise, when they must compose even fairly simple sentences rapidly and efficiently.
Whether he is characterized as dysgraphic (or SLD in written expression, which is another term for the same thing) also depends on some arbitrary factors: the DSM-5 now requires below age-normative performance for diagnosis, which eliminates quite a lot of 2e learners. This is not a position that the majority of the LD research/professional community actually agrees with. (But the DSM is a medical diagnostic manual, not an educational one.) Some states have also defined LDs as educationally-qualifying disabilities only if achievement is below grade-normative. Again, this is not universally agreed-on by the professional community.
So it may be that, in your specific situation, he does not nominally meet criteria for dysgraphia educationally or medically, based on your local conditions. This does not change the marked personal weakness he displays in expressing himself efficiently on paper, using complex and mechanically accurate language, which I and many prominent members of the LD research/professional community would consider symptomatic of a disorder of written expression. On some level, it also does not matter that much whether it arises out of his ADHD, or some other factor. Remediation and accommodation (other than the option of psychostimulants) will be essentially the same.
With regard to teasing out what aspects of writing are problematic, you may wish to observe his writing performance under different levels and types of scaffolding. For example, how does his writing differ when:
1. it is dictated/speech-to-text only,
2. he uses a graphic organizer or mindmap (with/without dictation),
3. he's provided with a sentence starter,
4. oral pre-writing/idea generation precedes writing,
5. spellcheck is on/off,
6. writing tasks are chunked,
7. those chunks are timed/untimed,
8. the task calls for sentence level vs paragraph-level vs multiparagraph writing,
9. tasks consist of informational vs literary (fictional/creative) writing.
On another note, you may also wish to contemplate the expectations for writing at the school to which he has applied, and whether they would be a good fit for his current performance in written expression, which is highly likely to be functioning much closer to the Average scores in sentence writing fluency and writing samples than it is to his exceptional isolated spelling skills. Looking back to your original inquiry, which included essay writing, I would be particularly concerned that his performance would be even more discrepant on a measure of extended writing, such as the TOWL-4, or the WIAT-III Essay Composition, both of which more closely reflect classroom writing demands than the narrower tasks do.
