I wonder what the ideal science acceleration look like.

Imagine it compressed where he is basically devouring 2-3 years of science a year. There will always be a topic he has already done a deep dive. Likely those are his favorite subjects, what happens when that topic comes up?

Or if it is more like individual tutoring, then there is only so much of a teacher's timeslice he can get.

If it is skipping into higher grade classes, as you say, the core skills (e.g. writing) may not be up to speed with how they do things.

Compacting? Without acceleration means less of the subject he loves.

Maybe the ideal is a grouping with like seven other kids at a similar level of interest and ability. But even in those cases, one kid's gonna be googly on physics, another on biology, etc.