I am EXACTLY where you are at with DYS DS6. It is NOT a comfortable place to be - on one hand, you want to teach him to respect and listen to the school and teacher. On the other hand, you know that your child is extremely unhappy because they are having to repeatedly do math that they have already mastered, perhaps long ago.
I personally think that 6-7 is MUCH too young to expect the average kiddo to advocate for themselves (and I have 2 DC with VERY different personalities, so I understand that YMMV).
Afterschooling works to a point, especially if it is limited to one subject. At some point, however, it is simply unfair to the child and the parent.
Does this school have a G&T program available now or in the future? Can your DS be subject accelerated (refer to the IAS)?
I haven't solved the problem yet. I am lending support, however, just so that you do not feel alone. I am also following this thread for the great suggestions of others.