Your son acts like a typical 6 year old, but definitely not like a 3rd grader
I can see why statements like that would make you cry! And I do completely understand your frustration. However, I think it *can* work (not ideally, but it can work). My ds8, a 3rd grader, goes to 5th grade math. And yeah, guess what, he probably acts like he's 8. We were very lucky last year to have him placed with a 4th grade teacher who had previously taught first grade, so she was very appropriate in her expectations. My feeling was not that my son should be given special treatment, but I did want a teacher who wouldn't expect 4th grade behavior.
For us, what's really worked is getting the group consensus. Last year, we met with not only the principal and his current teacher, but this year's teacher as well. I think it's important to have that meeting as soon as possible. This year, we met with his current teacher, his math teacher and his gifted teacher (scheduling, unfortunately, puts gifted and math into conflict, so like your son, he misses some). Yes, it would have been better to have it last year, but you really don't need her buy in (the teacher's) you only need her boss's buy in (the principal). If s/he is enthusiastic, that will go a LONG way in making this work.
We covered everyting in detail - when my son would miss class, how that would be handled, where homework would be posted, etc, etc... it was a bit easier for us because ds8's math teacher is my ds10's main homeroom teacher, so there is a level of familiarity and support.
One thing I really stuck to with all my kids (our two oldest were younger than their classmates) was making sure that it was an equal playing field. What I mean is - IF your son was 8 years old and displayed the exact same behavior, meaning, he's in 3rd grade and he's doing the same level of work and displaying the same behavior, would this teacher think that maybe he shouldn't be in 3rd grade math? OF COURSE NOT! Most likely, she'd be offering you workbooks and help and pushing you to work with him and make it successful. In essence, all you want is for him to be treated like the other third graders. But I feel like the teacher is just holding in her back pocket that he can "always go back" to second grade - instead of working her butt off to make sure that 3rd grade math works.
I would politely send a note to the principal, his current teacher and the math teacher and say, "hey, it's been a month and I'd like to evaluate how ds's plan is going. Specifically, I have the following questions/concerns. What day would be good to meet? I'd like to set aside 60 minutes."
Good luck!
Theresa