Many parents feel more comfortable bringing a doctor's note, but a note is completely unnecessary. Instead, you must be able to explain the type of assistance that you require.
Nine times out of ten, the Guest Relations Cast Member will ask you: What type of assitance do you require? This is an important question because even if the Cast Member knows your diagnosis and is well-informed, he/she may not know the type of assitance you require. Each individual guest will have different needs even if they have the same disability.
So, spend some time thinking about the assistance that you require. If your son is autistic some of the assistance that might be helpful for you might be:
--"He needs to remain in his stroller while waiting in lines"
--"He can't be in the direct sunlight for very long"
--"He can't wait in enclosed spaces with other guests"
Perhaps some of these suggestions will be appropriate. Perhaps not. If you can clearly state what assistance you would like from Walt Disney World, the Guest Relations Cast Member can easily provide that asistance (with very few exceptions).
Have a magical vacation.