I'm an adult Asperger's and SID diagnosed, and Disney is my "comfy spot" and greatest obsession. I'd love to offer some advice from my personal experiences.
Until my last trip, my DH and I had always either used Fastpass or skipped the attraction unless it was an indoor que with a less than 15 minute wait. I find that it's helpful for my piece of mind if I walk in front of him to control my distance from the people in front of me and he can control similarly behind me. Last visit I did bring in a doctor's letter and got a GAC that we used a few times when the Fastpass times were later than we would be at the park. Each time the CM in charge of the line was polite and discreet when assisting us, though we did have a couple of guests make rude comments because there doesn't appear to be anything wrong with me. My DH turned to one commenter and stated that he would happily trade places in line if I could be "normal" and not in the Autism Spectrum, and the commenter replied that "only children are autistic, everybody knows that".

Even when using the GAC we still waited in lines- just shorter or off to the side. It was a godsend to not have to be packed in and terrified, and it allowed me to relax a bit easier.
Don't hesitate to leave the park and go back to the room for some quiet time if your child shows signs of overstimulation. Some commando touring vacationers may think that we're crazy, but we often arrive early to a park, leave for a quiet resort restaurant lunch, go back for a little while to ride attractions while everyone else is at the parade, go back to the resort for a nap or to watch TV and be cool and quiet for a while, then have a resort dinner before going back in the evening for a few more rides or attractions. Setting our own slower pace and knowing that the quiet haven of the room is there whenever I need it goes a long way to prevent meltdowns. I'd also recommend chosing the timing of your attractions carefully if possible. Fantasy Land is ALWAYS going to be a nightmare during the day, but at night it's less populated and easier to manage.
Personally, I find that fireworks and parades are a bit too much in general. If it's possible, we may watch the evening MK parade from a less crowded spot like Frontier Square. I've only ever seen Wishes from the beach chairs at the Polynesian, which is far enough away from loud sounds and crowds that I can handle it. I highly recommend taking the resort boat to the Poly, having dessert at Kona cafe, and then sitting on the beach to watch Wishes and the Electric Water Pageant. It's a quiet safe environment and it still allows someone in the AS the abilty to watch the fun from a safe distance.
Other things that we do that might work for your child include taking boats for transportation whenever possible as they are often less crowded and slower paced than the monorail, deciding in advance on a designated shady and out-of-the-way spot in each park to go if I start to get upset or feel like I'm losing control, reading guidebooks and websites to plan our days in as much detail as possible well in advance of ever leaving home so that I know what to expect... but be willing to be flexible to accomodate any problems that may arise. For example, I've never visited the Tiki Room but always wanted to... and three minutes in I found it to be so loud and overwhelming that I had to be carried out and we had to find our quiet spot for a while so I could calm down again. Just because it looks like fun on paper doesn't mean that it will be as great in practical application.
Sorry to go on and on, but it's so rare for me to find another Aspie- and as you may know when we get fixated on something we really like to share our knowledge... and I'm a WDW addict

I think my DH and you parents/siblings/spouses of us neurologically atypical folks are all saints. I can't imagine having to deal with someone like me and still have fun on a trip.