Thank you for all the feedback. Here is some additional information, if it helps:

1) My son’s classmates are a pretty bright group. Most of their parents have advanced degrees and are scientists, engineers, doctors and lawyers. At least ⅓ of the class are gifted. Even with a VCI in the 140s and reading at a second grade level, my son isn’t in the top reading group. Also, his writing is just average so I suspect he may experience difficulty with keeping up with the written work of older kids if put into a class with them.
2 My son’s math level, however, is above any of his classmates’. I’m not sure how he compares to the kids in the other kindergarten class but from his teachers comments, I suspect he is much higher than anybody in his grade.
3) The school seems to be committed to differentiating. The kids’ reading groups are cross-class so that everybody can be appropriately matched. In older grades, the kids are tracked according to math ability and put in cross-grade classes. I understand that some fifth graders are even doing Trig.
4) The curriculum is project based, so math is integrated in all the activities and the kids aren’t given packets of work or asked to work alone. The school doesn’t use an off the shelf math curriculum but instead has a scope and sequence and a resource room where teachers can choose from a range of activities/games/lesson plans to cover each concept. According to the curriculum director, this allows teachers to tailor activities to the ability levels in a given class. Most of the math is done in small groups and consists of games and activities. Kids are also given individual, open ended activities at least once a week with the idea that kids can work at their own level. Then the kids share their work/reasoning with others. For example, when the kids were working on greater than and less than, my son wrote a series of math sentences comparing square roots of numbers. So he was able to take an open ended activity and make it more difficult/interesting. But he didn’t learn about square roots in his class, he learned about them from talking about math with his dad. Also, he is still spending most of his time playing the math games that other kids are doing.
5) While the kindergarten teachers in my son’s class are relatively inexperienced and also new to the school, the first grade teachers are veteran teachers. The lead math teacher for first grade said that math is “highly differentiated” in her classroom. She said that while kids are all working on projects together, she assigns parts of the project that provide different levels of challenge to various kids. I know a woman who co-taught with this teacher before and she reports that the teacher “bends over backwards to make sure kids are learning and challenged”. She said that some kids were doing very advanced math in the class and that all the kids were doing multiplication and division by the end of the year.
6) I suspect that while my son’s teachers call him “brilliant” or “the next Elon Musk” that they don’t really have any understanding of what he is capable of in math. I could share his IQ scores but if they don’t have much experience with gifted kids, I’m guessing that won’t really mean much to them and I’m afraid it will make me just seem obnoxious. I do believe my son has been challenged and learned so much in kindergarten, I just don’t think he’s being challenged in math.

All this is to say that I’m tempted to just see what happens BUT I’m also worried that my son will spend another year not learning anything in math. This week he told me that math is his favorite subject but it’s getting “ruined” because he never gets to learn anything new. He’s not particularly assertive so when he approached the head of the school to ask for more challenging math, I knew that he must be really unhappy with the current setup.

I feel like I want to get this right now because coming back to complain again next year will undoubtedly strain our relationship with the teachers. I am leaning towards asking the school to finish an assessment of where he is in relation to the current first grader’s math and also if there will be any kids who are similar in his class next year. I suspect that no amount of differentiation would be as good as being paired with at least a couple of other kids who are developmentally matched with my son’s math level.