I've never needed a note. When you arrive at the first park you visit during your stay, go to Guest Relations. Explain to the Cast Member what your specific needs are (what you stated here is perfect for them, honestly). They will ask how long you will be visiting and how many in your party.
Okay, he's got PDD. Some suggestions:
1: If he'll tolerate a hat, it'll really help to cut down the excess stimuli.
2: You don't state his age, but if he's young enough to be in a stroller, bring a towel or blanket that can cover parts of the WDW strollers/your stroller and again, cut down on the stimuli.
3. If he's bothered by heat, see if you can track down some comfort cools. They are wraps that you soak in water and then wrap around your neck to help keep you cool. These are iffy with my son, but when it's really hot, he'll tolerate it because he knows they help.
4. If he's old enough to use a camera, the disposable ones are a great distraction device, especially during the waits for a parade. A friend gave both of my boys cameras with instructions to bring her back pictures. My younger son had great composition for a 3yo, and my older son showed his potential future career as a PI with all the sneaky shots!
5. Your hotel pool can be your BEST friend. Over time, we learned that either day three or day four, the entire morning was spent at the pool. (this is when we travelled to WDW twice a year prior to moving to FL)
6. Go at his pace. Our first trip was great because of the newness. The second trip was right after my sons DX (Aspergers, ADHD, OCD, SID, ODD)and it was hell. Granted, we did not know about the GAC then.
7. If he's a picky eater, bring some snacks/drinks with you that he will eat. We first started with a fridge in the room, then bought a refrigerated cooler. It was stocked with yogurts and drinks he liked. I'd put crackers, raisins and other favorites into the backpack. That way, if nothing on the menu appealed, he'd have *something* and eventually, somewhere in the park we'd get something my picky guy would eat.
Enjoy your trip!
Suzanne