Options look to me like:
--get outside achievement testing to demonstrate what level he's working at
--look at your district's policy on grade acceleration. (Here they have to pass end-of-grade tests for the next grade up.)
--wait it out. It sounds as though he's not unhappy where he is. Kindergarten is still a very social time, learning the ropes of school; the academic aspirations are usually not too rigorous. If he's way ahead in first, they are more likely to want to address it.
If he's not terribly unhappy, I'd probably wait it out. YMMV...
DeeDee
