We have a similar situation with DS10, who is 2E/Asperger's as well. He was 9 at the start of this school year, going into 5th grade, and at the moment he is in 5th, 6th and 7th grade classes, mostly 6th. He is officially a 5th grader but will skip to 7th next year. We're lucky with our school people, as well, although there is occasionally a question of how long it's going to last when he is in MS/HS only. Fortunately for us, there is no bussing involved, as the whole K-12 is on the same campus. Just a lot of walking and being late for things.
We find that he does better socially with the older kids, and when he is working closer to his levels academically. The more bored he is with the work, the worse he behaves. It's a fine line for him, though, because he also has lazy habits from never really being challenged, and he gets upset when he doesn't know something.
It sounds like you have a school district much like ours, which is a great and wonderful thing, and so rare. I'd say keep doing what you're doing until it doesn't work anymore.
