Unfortunately, I'm not sure there's much benefit to skipping/acceleration for science and/or social studies in the elementary years. Keep a long term perspective.
In my experience though, 4th grade science, 5th grade science, etc...is all just "variations of the same". The expectations are probably a bit more in-depth with each passing year, but without a true end goal (or an outright grade acceleration), I would try focusing more on depth than distance. Whatever each grade is already presenting can for certain be expanded and broadened and "taken to the next level". (This is not always true with math, especially in the earlier years.)
I totally agree on the social studies and science. That's what I want.
I do think there is Endless alternate Math for enrichment that could be used with the extra time available if he tested out or condensed.
I think a Math & Science in one course combo could be the ultimate solution or something like art of problem solving, math counts type work or computer programming.
