In answer to your much earlier question about fine motor: while it is true that all the scores are at least within normal limits, there is a huge disparity between his average motor coordination and fine motor skills and his very high visual perceptual skills, which suggests that there is some impediment to his demonstration of his exceptional perceptual skills when fine motor output is required. Perhaps this is a more nuanced interpretation, and one that some OTs would not accept as justification for supports, since they tend to see relatively narrow slices of a child's functional profile.
On additional writing assessments: I like the PAL-II for this age, as it distinguishes between various levels of writing complexity, including fluency.
The good thing about his interest in the big ideas of science and history is that these are relatively easy to supplement, with educational videos, books, and fun science websites. Coding and reading music actually aren't that hard to do for fun, either (don't know his level of coding, but Scratch is fun and age-appropriate). Reading music is just a matter of practice. If you can find middle C between the staves, you can figure out everything else. Rhythm notation is simple fractions.
The hardest thing to accommodate outside of school (without going full home/after school!) is probably the level of math instruction he'll need. My personal inclination would be to prioritize (among academic needs; obviously his holistic happiness is more important than any one academic subject) appropriate math instruction, and let the other chips fall where they may. He's only going to be in second grade, so there's plenty of time for higher-level structured science and history instruction in the future, just as long as the current level is not so low it frustrates him. At his reading level, he has access to a pretty rich range of books on any topic that interests him.
Hold on to your sanity...I think you'll all be fine!
