If a bit more than 11% if the kids in his grade are functioning at the same level as is he, I would be more inclined to get the parents of these other kids to join you in advocating for a cluster grouping like others have mentioned where the entire group uses the 3rd grade math curriculum & has more challenging and deep work in other areas. We did something like that for my youngest toward the end of elementary & it worked well b/c there were enough kids to create a group to work a year ahead in math & reading without asking a teacher to do something different for just one child. I also think that makes more sense than having him miss another core subject to take 3rd grade math.
Grade skips, to me at least, are warranted the most not just when a child is an excellent candidate on the IAS, but when s/he is an excellent candidate who has few or no peers in grade and whose academic needs will never be close to met without the skip. Especially since you anticipate school being a better fit once he gets beyond elementary, I'd try to get the elementary to do something more for the gifted kids. BTW, I am not anti-skip in general and do have one other child who is grade skipped.
