Does he excel in other classes, too?

If so, you should think about grade skipping.

Busing to middle school for 2 years will disrupt a lot of classes. My DD skipped kinder and took the State tests (6th, 7th and 8th grades Math) after 5th grade to do Algebra 1 in 6th grade. So, she did not have to bus to middle school. She still need to bus to HS for Algebra 2 next year. She will have 2 other kids from the same MS going with her otherwise we would not go for it.