Welcome Back Debra ~
As a Mom to a son with cognitive and mobility disabilities, I love to see that you are planning ahead for your son.
The Walt Disney World Resort is committed to assisting Guests with all of types of special needs including those with
cognitive disabilities. They have put together "A Resource for Guests with Cognitive Disabilities including Autism Spectrum Disorder (ASD)" and "Attraction Details for Guests with Cognitive Disabilities". You can download both of these online by clicking on the link I've provided for you. That will give you some great information that will help you before and during your trip.
When you reach the seating area for Fantasmic!, speak with a Cast Member and explain your situation. They should be able to help you locate space at the back of the amphitheater. That way you can make a quick exit if necessary.
I would also suggest requesting
Disability Access Service (DAS) for your son by visiting Guest Relations at any of the four theme parks. Cast Members will ask you some questions so that they can best accommodate your son's specific needs. You will not be required to provide any type of supporting documentation to the Cast Members.
If your son has any comfort items including electronics, bring them if possible. If he has headphones or ear plugs to help keep the noise level down, bring them. If he becomes anxious, remember to relax and remove him from the situation until he calms down. There are quiet places in the parks to give him a break. If you see that he has had enough, pack it up and head back to your resort. You may not get to do everything on this trip but you can make the most of everything you do.
Wishing you minnie magical memories :-) Allison V.