Welcome Back Jessica,
You will be allowed to bring in your son's special toy as long as it isn't on the list of items not allowed in the theme parks. Click
here and scroll down to see a list of items that aren't allowed to be brought in to the theme parks. No documentation is required as long as it isn't on the list.
Keep in mind that you will have to keep up with it all day so make sure you plan for that (i.e., is it heavy or bulky?). If it requires batteries, bring extra. If you forget and need them, you can find batteries at a lot of merchandise locations. You may want to put your name and cell number on it just in case you misplace it. If he has two, you may want to keep a spare in your room.
For those times when we're waiting for parades, fireworks, etc., I usually take some bubbles with me. Not only does it entertain my boys, but we make some new friends as well.
The last part of your question got cut off so if you need anything else, you know where to find me :-) I'm super happy to help you!
Wishing you many magical memories :-) Allison