Hey, I'm not a teacher, but was more or less, "that kid" if i sound arrogant i don't mean to I just had a really hard time in high school. I don't have any sort of autism though so your son is head and shoulders above where i was just due to that.
I was the smart kid in all the gifted and talented classes until about 8th grade. At that point the teachers stopped challenging me so i stopped doing the work. Instead of pushing to get the most they just sort of left me hanging out doing the same work as everyone else in half the time and then left to amuse my self for the extra 10 minutes. I would read books or color. Nothing disruptive since I was quiet and shy, but I'd get in trouble for "distracting other students" or "leading rebellions"
Not saying I was doing the right thing, but i'm not a sit still at your desk and wait for everyone else to finish type person. I ended up being tutored in math over the summer so that i could skip a grade level for the challenge. and it became a HUGE issue with the school system. Alot of other random stuff happened in that time period but needless to say, It turned into this huge battle over me between teachers and administration and my parents. in the end my parents put me in a private highschool starting my ninth grade year. That solved alot of the problems for me.
Just wanted to let you know that if your son has a good track record than fight for it if its what he needs, but don't let the fight get in the way of his well being. Good luck and those of us with issues do turn out ok if given the chance! I'll keep you guys in my thoughts.
(thats me cheering you on)