One of the things I brought when we were in a similar situation was a letter from the preschool (in your case, pre-K) teacher going into a little bit of detail about both academic and social readiness for K and their thoughts that my child would do well going to K early.
Another thing that might be useful would be for you to get ahold of the district guidelines for skills readiness for K as well as curriculum goals for each grade level. Often, these can be found on the district's website. That way, you show knowledge of the curriculum and can show were your DS's skills lie within the curriculum.
Also, one thing that helped both me and the school was for my child to visit a K classroom at the school for a morning or afternoon to gauge whether your child has the ability to separate from you, listen to directions, engage himself appropriately during free time, and compare him to the other kids that are, admittedly, a year ahead in school. But, if he compares favorably, this can go a long way in alaying any fears.
Hope this helps. Good luck!
