Specifically, I would like to advocate for my son to either: accelerate in math by 2 grades or allow a small group to form in his grade and bring in another teacher to teach the group.
I suspect that you'd have more luck asking for a single grade subject acceleration in math to start rather than two grades of subject acceleration for a few reasons:
1) schools often want to try out something less extreme to start to make sure that it works before they agree to more radical measures. If they aren't in the habit of providing subject acceleration at this point, anything like that might feel radical to them.
2) if he scored around the 50th percentile for a 4th grader in math on the winter norms for MAPs, that would be about the 99th percentile for a 1st grader, 97th for a 2nd grader, and 80th for a 3rd grader. I think that it would be easier to make a case that he would still be quite near the top of the class in a 2nd grade math class and not missing any pieces. Schools can be a bit worried about gaps in math education when multiple skips happen all at once and the difference between 2nd and 3rd grade math is greater than the difference between 1st and 2nd grade math.
Like the pp, I do also think that you'll be in a stronger position to advocate once you have his WISC scores and also if you have any idea as to whether there are other kids scoring near the same point on MAPs such that they can make a reasonable group within his grade if that is something that they are more willing to consider over subject acceleration.