xdarkroastx,
Having been on both sides of the table in this type of conversation, I understand both the frustration and the constraints. (Actually, I've been an advocate for acceleration on the school-based team side of the conversation, too.) One of the challenges on the school resource side is that the teachers who are excited about his gifts and interest in numbers -and- have the skills to differentiate/accelerate instruction for him are not as plentiful as one might think. His existing teacher sounds delightful and caring, but may or may not truly be equipped to keep pace with your DC's needs in math instruction.
A possible direction that may cost the district money (or not, depending on what they already have), but would not require a teacher, is a self-paced online platform such as ALEKS, which has instructional modules embedded in it, is self-leveling (through benchmark testing at the beginning, and ongoing adaptive assessment), and includes content from kindergarten through calculus. (ALEKS Adventure is more elementary-appropriate in its aesthetic and format, but only goes through 5th grade--which might still be useful to your DC for a little bit, while the main ALEKS package is grade 3-12 content and tends to be more of "just the math", which might work for a kid who actually likes math itself.)
The advantage to a strategy like this is that it is designed to fill gaps (which is often one of the objections to acceleration, compacting and/or skipping), and also has a much higher ceiling than is typically available to an elementary school teacher (or even, TBH, most high school math teachers). There are also crosswalk documents that align and correlate the topics in ALEKS to your state curriculum frameworks by grade and by secondary-school course, so the GT teacher should have no difficulty identifying where your DC is with respect to state requirements and expectations. It's designed firstly for institutional schools, so it has all the dashboards and teacher monitoring features that districts want, but it's also (FWIW) available to parents as a home subscription, if it turns out that you aren't able to get movement from the school, but want to give your DC a more structured option for self-paced learning. There's a free trial version that lasts a couple of weeks (I think), which would be a way to try it out, and maybe collect some data on present levels and rate of learning. I frequently recommend it to my homeschooling people if they are struggling with their existing math curriculum.
Free options that have some of the same self-paced options include Khan Academy and IXL. Khan Academy is more systematic, with brief instructional videos, but equally matter-of-fact, while IXL is more game-based, less comprehensive in content, and lacks embedded instruction.
Another option might be to discuss the possibility of cross-grading for math only, but that will likely depend on buy-in not only from administrators and GT staff, but also from both the sending and receiving teachers. It also depends on how well he would be able to meet the behavioral and executive function expectations of the receiving classroom, which tends to figure into the receptiveness of the district to this as a solution (even though, yes, it is related to his disability, and no, it has nothing to do with his math instructional needs). Plus, if the schedules can be made to align. If everything else works for this, the upside to this option is that all of the adults just do what they were going to do anyway. The only change for them is the physical location of your DC during math.
You'll notice that the theme across the solutions I've proposed so far is that they are minimally inconvenient to the adults involved. This is, of course, unfair to your DC, who (rightfully) is your priority, but it is also eminently practical. One of the contributors to successful advocacy is communicating not only your child's needs, and specific solutions (i.e., not just complaining, but offering answers), but also conveying your sympathetic awareness of the challenges and resource constraints that teachers and schools deal with. It is also both savvy and gracious to acknowledge and express appreciation for anything that they are doing well--or even simply intending well, no matter how small it might seem.
indigo has collated many thoughtful posts regarding advocacy; if you haven't yet had a chance to do so, I would strongly suggest searching back through the posts (try "advocacy as a non-newtonian fluid"--that's one of my personal favorite thread titles!) for much excellent advice and anecdotally-acquired wisdom.
