Hi Song - we also have a DS5 going into kindy in the fall. We first approached the school's gifted coordinator (who really works with kids starting at 3rd grade), which was very helpful because she really got what we were talking about. If your school has a gifted coordinator/teacher (even at the wrong grade level), that person should be able to tell you about the school's policies. (You can also try to search out your school's policies on the internet or at your local library - i hear many school's have the policies available in those locations). At the same time we contacted the gifted coordinator, i also contacted one of the state's gifted specialists, who i found on the state's DOE website.
We already had IQ testing under our belt, so that helped us when talking with school personnel. They got a report from a professional, versus hearing from just another parent. Along with the report, what helped us was approaching the school with an attitude of "we don't know what we should do, our kid is like this, what do you recommend?" The school came up with the idea of achievement testing then.
Since it sounds like you don't have testing available from your post, maybe you could get a letter from the preschool teacher that states how advanced she feels your DS is compared with other kids, and approach the school with that and your descriptions of what your child can do (if you have anything you can put in a portfolio to show the school, that would probably help too).
Good luck! (fyi - our school offered differentiation in the kindergarten classroom for our DS, along with handpicking a kindergaten teacher who was willing and able to work with him, and offered regular meetings to see if the differentiation is working. We didn't advocate for a skip, since our DS is going to have a big adjustment going from very parttime preschool to full-time kindy anyway, and also he doesn't write yet.)
