These may already be in play/have been tried, but here are a couple of thoughts:

1) For worksheets, would he be willing to do four problems? I'm thinking about Weinbrenner's "most difficult first" strategy in which a student who can do the four most difficult problems on an assignment is then excused from the rest of the assignment (having thereby proved they have the concept and don't need the repetition).

2) I wonder about the idea of "not ready" when it comes to giftedness. If he is interested in taking on a much higher level of math, perhaps he would be able/willing to work on the pre-requisite skills as he needs them. I would think he would have to be pretty highly gifted in order to do that, but perhaps that is true of him?

3) Can the interests he has be harnessed in order to work on the areas of work he is resisting? For instance, if he works on an independent study related to his interest in medevial times, can he work on a project that involves mathematical calculation? Or can it be more fully integrated into the science topics/study he is interested in pursuing?

Obviously I don't know your child, but as I read your post, I started thinking about that Stephanie Tolan piece, "Is It A Cheetah?". Do you perhaps have a highly or profoundly gifted child who is with a group of moderately gifted children?

I think you mentioned that one option is to have the gifted teacher mentor him. Presumably this would be 1:1? Is there a reason that that would not be a good option?

I was also wondering about the writing in cursive piece. Is he not allowed to turn work in that is written in cursive? I think that it is not unusual for highly able children to resist writing. Their motor skills can't keep up with their intellectual speed, and it can make it pretty frustrating to share knowledge in written form.

Good luck--I'm sure that the smirking is frustrating for you and for his teachers. I wish I had some good advice for you there...
Tam