Thank you, Tigerle. I do think keeping in mind "least worst" may be key. This is going to be a long road.
A couple of developments since I last wrote:
a) The school decided they are more comfortable with a single-grade math acceleration for now (he showed fatigue both in general and especially in handwriting during his evaluation, so they want to take this in easier steps, plus the math times line up between K and 1 so it would be easy to implement). This is OK with us for now, as my son is intimidated by the idea of being with older kids. This will start next week, so we'll see how it goes.
b) The school initiated a request for a full gifted evaluation, including individual IQ and achievement testing and various grade-level assessments, among other things. This will lead to a meeting in which we can discuss appropriate accommodations (possibly including a full-grade skip), but with a lot more data in hand.
I feel pretty good about his situation for the next couple of months. He'll get more challenge right away and I know I'll feel much more confident making longer-term decisions once we have more data in hand. I have to say, I'm really impressed with how willing and even eager the school seems to be to help him. All of our explorations and conversations with the school about gifted services in K (before they met our son) led to the conclusion that a) they only do in-classroom differentiation in K, and b) they never assess students in K unless the parent requests it, in which case they have to by law. So, their response has been a very pleasant surprise!
