The only real thing I see to do is to apply for ds to be assessed for identification in math for the gt program, so that they are 'forced' to give him differentiated material. I know that for folks around here with pg kids and problems specific to that, this might sound like a minor problem, but imo this might to make the difference between ds hating math forever and loving math forever.
No, I don't think that you sound overly aggressive at all. We were in the exact same spot with dd9 this year. She, too, is a 5th grader (will be 10 soon) and we were done with her not getting what she needs in math. She isn't as HG as some of the kids here either, but she has gifted IQ scores on the WISC-IV, scored in the 98th percentile for math on the WIAT at the end of 3rd grade after being taught virtually nothing in math that year, and scored high enough on the math part of the EXPLORE in 4th grade to be considered gifted for a 5th grader. B/c her group test scores are erratic (all higher than average, but some not high enough), she was going to be in straight 5th grade math this year with nearly 30 other kids.
It was an issue of her self image and her liking math. I see math ability in my dd b/c she is so creative with how she approaches math. She creates new ways to solve problems that work such as dividing and finding the area of a triangle. She knows the answers without knowing the intermediary steps that I have to go through. She just leaps from one to the end.
We were done so I emailed the GT teacher and the principal and asked for them to reconsider her math placement. We were prepared to homeschool as an alternative. Dd is now placed in the GT math class where they are doing 6th grade math and the GT teacher told me in the parent conference that dd is one of her stronger students. She probably won't be an A student on all of the material in the class, but she is holding her own and she
understands the why of things. More importantly, her self image and math confidence is in a totally different place which is showing in her group achievement scores finally, too.
Good luck to you and your ds! I hope that you can get him what he needs.