I would suggest getting your hands on your local school districts learning targets for 4K - 3rd grade. When you observe mastery of a skill, check it off with the date and keep a piece of evidence.
I wouldn't list activities, because the school might come back and say he's not advanced but has benefitted from "enriched environment". The reason you'd hear that is hot-housed toddlers typically regress to the mean by 2nd grade -- and districts save money on GT programs by withholding them until 2nd, 3rd, or even 4th grades in order to weed out the kids that will regress to the mean.
I would find out what measure of reading your local district uses, and communicate his reading level in their terms. Rather than an exhaustive book list, state "DC reads at approximately Lexile XXXX, as demonstrated by this list of books he has read in the last month" Even better, use readtheory.org website for free reading comprehension tests and corresponding grade and Lexile levels.
If your state has public 4K, start looking at the enrollment specifics for early enrollment, check out Kindergarten and 1st grade enrollment requirements too. Elementary years tend to be the most difficult for academic match - and I think the most damaging too. Start figuring out the loopholes and local educational system, shadowing schools, attending school board meetings, volunteering in the school too. The more you understand about the inner workings, the better an advocate you can be. I think knowing what they can and cannot accomodate helps before you start asking for modifications of their program.
