Your son sounds a lot like my DD6 (just had her b-day!) academicaly. I don't know if the same would work for your son, but by our experience this is what we did for her.
To start we put her in K (private school) then the teacher suggested we have her tested. (you've already done this so you are way ahead of me) in Jan. we put her in 1st grade. This seem to work for her because she had not been in any type of pre-school so things were new and exciting to begin with.
Next year she will be going to a different school, with higher academics and the ability to go beyond what they are learning in the classroom. I can see she had perfection issues so I always want to make sure there is a challenge in her school work.
I don't agree with the principal about these kids not knowing the concepts, my DD just seems to know or she reads about it and figures it out, I think that's one of the reasons why they call them gifted?
I think you just have to take it one year at a time, find what works and be pro-active about their education.

Sounds like your already going that direction.