I see your son's reaction as a mature response to a total lack of stimulation from the teacher. He's exhibiting, IMHO, a healthy balance between setting stretch goals for himself and avoiding perfectionism. Wonderful!
I would continue to offer as many after school opportunities as his interest and enjoyment allow while continuing to push for enrichment or acceleration. In doing so, you're giving him a satisfying opportunity to achieve near his potential! Your intuition is on point.
You might wish to share the following article with the teacher. She clearly isn't making an effort to understand your son's frustration at the stultifying pace of learning. A little perspective taking on her part could go a long way.
This might be somewhat hyperbolic, but I see holding gifted children back to average achievement levels as similar to mentally putting them in prison. It makes my blood boil when good, hard-working children like your son are discouraged from learning. A non-gifted student wouldn't be put in a class for the mentally disabled and expected to make do.
Teachers like this make me want to retrain in gifted education to shake up the system.
http://www.stephanietolan.com/is_it_a_cheetah.htm
