I f you just want to watch the fireworks of Wishes without necessarily hearing the music, the fireworks themselves are high and you can view them from any of the MK resorts, along the side that faces MK. A place in MK you could try would be the second level porch of the train station at Main Street (we haven't tried this, but I think you would see a lot since you would be facing Cinderella's castle where most of the action takes place). Other than that, I don't know, but hopefully you will get some other suggestions.
For Illuminations, some of the "action" takes place on the water in WS Lagoon. The highest fireworks can be seen in the air from locations at the Epcot resorts and even as far away as Caribbean Beach and OKW. But, those high fireworks are just a small part of the show. One place in Epcot that is removed a bit from the action is the area right in front of First Aid (look on the park maps for Odyssey Center. There is a walkway that goes between that and WS and between that and Test Track. You can see pretty well from the walkways, but there will hardly be any people and you are somewhat removed from the fireworks. You can still hear them, but they are not so loud. Also, there is a handy set of bathrooms right there where your person with autism can duck in if it becomes too stimulating.