I don't have experience and hindsight on this but the scenario you describe is one I fear down the road. I would personally start by having my child taking a break from school, while I work at the adult level, much like what Grinity suggested. A few days out of school with some firm messaging to the principal or teacher may shake some things loose for the remainder of the year, as it really is a temporary situation. In the meanwhile your son can get to enjoy math on his terms.
The test results may or may not help, as the teacher or administration may have already dug in their heels on this. You'll know during the few days off if this is the case...
