I know you posted this in December, but I am answering your post just in case you are still suffering with this issue. I also have my oldest son in kindergarten, he is from January 2005 so he is already 6 years old. I had his first conference (Teacher-Parent) in November and his teacher recommended moving him half day to first grade, but he only goes to school for three hours therefore he goes to first grade for only like an hour each day. He is also really advanced in Math, but because he is an English Learner (we only speak Spanish at home) he stays in kindergarten during Math so he can learn the vocabulary related to this science.
I have to mention that at his school all of the classes, through the different grades, have Language Arts at the same time, which makes this acceleration easier.
I did not have to say anything to his teacher during our conference, because she mentioned first but I was also feeling that he wasn't being challenged enough for his intellectual capacity. I would like you to know that his first grade teacher send notes telling me that he is doing great, and getting 100% on the spelling and comprehension tests that he is taking in her class. I feel this is going to be a long road, but I am glad his teachers are being so understandable. Good luck with your son!!