polarbear - It might not seem like it but I do believe that the teachers have DS's best interests at heart.
Maybe I'm jaded, but after reading your messages, I wonder if they're focusing on what they feel like focusing on instead of what's best for your son.
Many educators I've met are clueless about mathematics and clueless about giftedness. Part of the evidence for this is the way they stick to faddish math programs and the way that they cling to the idea that if your kid hasn't been through their school's fourth grade math book, he can't
possibly understand long division. More evidence: too many of them disparage approaches that are favored by mathematicians, physicists, and engineers, and fail to even consider that a kindergartner might be capable of doing second grade math. IMO, this is because they simply can't see past their preconceptions, even in the face of evidence. The kid got an A on the assessment exam? Yeah, but didn't get 100%; therefore he failed. And if he'd got 100%, he didn't use
our method. Therefore, he doesn't
really understand addition, and he failed. (Sorry Polarbear, I just can't laugh at this stuff. It's too destructive.)
Bottom line: you can't convince people who aren't interested in listening.

Yes, teachers know way more about how to deal with a heterogeneous classroom than most parents. But this doesn't give them expertise in mathematics --- yet many behave as though it does. I mean, your husband is a mathematician and they question how you and he approach math with your son?
Seriously?
I will go back again but I can't tell you how difficult I find this sometimes. Actually I'm sure you know in a BTDT kind of way. This is not my personality at all and at times I am too worried about offending that I find it hard to make sure that I am understood.
This I understand. It's a really tough position.
We solved the problem by embracing the suck and accepting the reality of the least-worst option. This means afterschooling and summer vacation math. My kids don't really complain about it, either. We've found that 30 minutes or so 3 or 4 times per week is enough to allow our kids to move forward at a pace that suits them. We ignore the school math, and I've even been known to complete a worksheet of basic addition while the kid does long division or fractions or whatever.
The result is a (homeschooled as of this year) 12-year-old who's excelling in Algebra 2, learning to apply basic trigonometry in physics, and who scored at CTY's award level on their placement exam when competing with kids two years older than him. It's been a lot of work with him, but it's worth it, IMO.
Good luck. Sorry you have to suffer. At least you know you aren't alone.