Thanks for all the input! Lots of good points. Hmmm.... I guess I don't know how ADHD is diagnosed. If it's based on a teacher's comparison with other students who happen to be older because a child is accelerated, then the immaturity factor definitely makes sense.
Luckily, the school that DS will be attending has a small class size for kindergarten (max of 12, which is smaller than what he currently has for preschool) and it's a gifted-only school with teachers who are trained for working with gifted children. They are particularly tailored to 2E students and those who might not do well in a traditional school setting, and they are extremely inclusive. So, given all those factors, it seems like a good idea to go ahead and start.