I'll be watching this thread with interest. My son is in K. Totally happy. I know K is 2yrs beneath where he is but I'm inclined to follow "if it ain't broke, don't fix it" and just continue to afterschool. Should he become dissatisfied, I will then go to the teacher.
On the other hand, you don't want them to learn that school isn't a place to learn new things. But if your son stands a good chance of getting a great teacher next year in a 2/3rd combo class AND he is happy now, I'd be inclined to let it go and continue ASing for now.
I had one kid who hated K, totally miserable, absolutely abhorred cutting, pasting, coloring - loves to draw his own stuff but not coloring sheets. I talked to the teacher to no avail. I'm still dealing w/ issues from that horrid year and he's in 3rd grade and being homeschooled. So for now, I'm enjoying my Ker being excited every morning to go to school and coming home happy rather than totally deflated.