Welcome! Let me start by saying that I'm not familiar with the US system and I'm sure someone who is will be along in a minute... However, a quick google took me to
this report and if it's relevant and I'm reading it right, your DS is not only above the 99th percentile for K, he's also up there (90s, anyway, depending on exactly which table you read) for end-of-1st graders (and still looking pretty good compared against 2nd graders come to that). Of course what you really want to know is what the 1st graders at his school are getting; but if he's really already mastered the reading and maths they're expecting him to learn in 1st grade, it's possible that a grade skip is more workable than differentiation (where it's easy to promise, and hard to deliver, as you know). How's his writing? That's often the limiting factor.
