That would've been awful for him, also!!! What kind of things would you suggest to remain calm at Magic Kingdom? We do try to find somewhere to just sit and relax (even on the ground) or go to the hotel to swim if it's too bad. Just wondering what ways we can help him when he does get overstimulated. Thanks so much!
Actually my advice would be to make an effort to avoid over-stimulation. Taking deliberate 5-20min breaks between exciting rides and load places can prevent having to leave the parks later on. Being over-stimulated sucks. Even after you "get better" the stress of it is still there and will just add up as the day goes on.
Think of it like a stimulation meter. When it hits a certain point you break down. You want to take steps to continuously lower that meter throughout the day. This is important not just at Disney, but at home too. Ask him how he's doing. If he's bad at describing it, get/make a feeling chart. Both of you should become aware of how he starts to feel before he gets over-stimulated to you can take steps to prevent it all together.
As far as some actual tips:
1. I'd get some noise-canceling headphones if you don't have them already, it's nice to have the option to shut out the world wherever you are. If he likes music, get the headphones that you can place sound through. It may help to learn some basic sign-language as well, if it turns out he really likes them. If I go to the parks alone, I'll usually just wear the headphones all day as the cast members are really good about using different methods to communicate.
2. Since he likes RC cars, I'd bring a small one. Like around 6in small. There are a couple places in the park that remain pretty quiet even on busy days(like the service dog area behind the circus) and he could zip it around for a little bit to relax.
3. A paper bag. Yes I know, seems silly. But I find that when I'm really anxious, a good bit of C02 forces my brain to slow down and think. Holding his breath can also work, but it might take a few tries to get it built up enough unless he's really good at it.
4. Stimming jewlery. I make my own chainmaille stuff, but you can find some online. There are also ones that are safe to chew on. If I'm waiting in line I'll pop mine off and start messing with it to help get some of the excess energy out without flailing around and bringing attention to myself.