Argh, I lost what I wrote. Short and sweet - I would keep pushing, were it me. Your DS sounds similar to mine in terms of personality and interests. A year is a long time in an 8yo's life, and he's asking for more challenge.
Could you work with the teacher or administration to get him accelerated to 3rd grade math? I don't understand why schools are so against it - instead of having to worry about differentiating in the classroom or having a parent on their case, the teacher can send the child off to another teacher for an hour and solve the problem. It works wonderfully (at least, in our case).
What about simply not turning in the homework, since it's not graded?
I'd talk with the teacher one more time and let her know your son really wants more math challenge, here are your suggested solutions, and let her know gently that you will talk to the administration about accelerating him, or whatever it is that you'd like to do, if she's not able to work with you on a solution. It can be done in a collaborative manner, while still making your point.
