You've received wonderful advice above, so I don't have much to add! One thing I'd keep in mind is to try to keep your letter brief. You'll have documents to back up your request - test results, letters from pre-school teachers, etc - you don't need to repeat what these tests and letters say in your letter (hope that makes sense!)
Stress a bit more that he is physically advanced as well, very tall for his age and hopefully as fast and coordinated as the other rising kindergarteners. Don't hide it in a list, make it an extra sentence or even paragraph. Again, something educators like to see.
While I agree educators might be interested in seeing this, I see it more as an argumentative point which the school staff might throw back as a reason not to accelerate than a reason *to* accelerate. Your letter should be about the reasons your ds needs to be in early K, not an additional list of answers to anticipated reasons not to put him in (hope this makes sense!). If he's tall and physically advanced, that's great - but save your notes about that for the actual discussion rather than this letter.
As others have mentioned, I also wouldn't state that not accelerating would result in some negative outcome - unless you have tangible data/proof that it would. Instead, focus on the positives of why an acceleration is appropriate and why it will work.
Stress his good graphomotor skills, mention what his teachers say about it in relation to the rising kindergarteners - don't just use extremely or excellent all the time, put it in perspective.
I would include this info, although I would include it by including samples of his writing/drawings/etc. A teacher isn't going to necessarily be impressed by a parent saying her ds has excellent graphomotor skills, but they'll pay attention if they actually *see* it.
Best wishes,
polarbear
