If it makes you feel more comfortable to have one, you can bring a note.
It hold say what your child's needs are, not the diagnosis. And be prepared that Cast Members will probably not want to look at it.
You will need to be prated to talk bout your child's needs and bring him with when you request the Guest Assistance Card.
There is more information in post 6 of the disABILITIES FAQs thread, which you can find near the top of this board or follow tHe link in my signature.