I'm with the others in urging you not to send that email. Wrightslaw does educational advocacy, mostly for children with disabilities and 2E, but I think the ideas from this "letter to a stranger" might help you reframe how you want to deal with the school in order to get them to meet your son's needs. It discusses how to get the school on your side.
http://www.wrightslaw.com/advoc/articles/Letter_to_Stranger.htmlYour letter should probably reference your state law pertaining to the rights of gifted students, and ask for a meeting to discuss how the school is meeting your child's needs based on the state law.
I would definite express concern that he is doing the same math he did last year. But it's so late in the year-- can you create a plan to challenge him through the year? Will he be ready for 4th grade math next year, since he basically hasn't had any new math instruction for an entire year? (That is such an unacceptable situation! Even if math schedules didn't match up with the 3rd grade, he should have been doing 3rd grade math.)