Hello! I am also a volunteer wish granter with MAW! I'm only one year and two wishes in, but I did complete the training this past October so hopefully I can help! This spring I had a 16yo boy whose wish I was granting. I wasn't told in advance, but he was pretty much non-verbal. I would say his communication skills were on par with a two year old. He could answer questions, but only in one word. Also, we asked the same question multiple times and his answer would change.
Before you get a wish granter, you will get an "About Me" paper to fill out. You may have already did it at this point. This has simple questions like "my favorite food is..., my favorite animal is... my favorite color is..." This will give the wish granters an idea about what your son is like and what he likes. In my 16yo's case, almost every answer was related to animals. He had multiple pets, his favorite activities were fishing and going to the zoo, his favorite TV shows and movies were the "Planet Earth" documentaries. So, going in we knew that his wish would probably be animal themed. At this point, when a child is so young and nonverbal, we need to trust the parents. Luckily, this guy's parents were amazing. They were able to tell us about past vacations they took, and what "J" did and didn't like. In the end, we went him to Alaska so he could do a bunch of animal and nature related activities like go whale watching, see glaciers, visit an aquarium, see sled dogs, and visit a wildlife sanctuary. I should mention that this family also traveled a lot. We suggested visiting places like the San Diego Zoo, the Smithsonian Zoo, and SeaWorld, but he had already done those before, so we wanted to give him a new experience.
In this case, the wish granters are really going to have to trust you. You know your son the best, and you can advocate about what he loves. Wish granters play the "wish game" with kids, so it can certainly help you and your son out. Wishes are usually categorized into four types of wishes: "I wish to have," "I wish to meet," "I wish to go," and "I wish to be." For each of those four categories, the wish granters will try to get three ideas. For example, they might say "If you could go to any place in the world, which three places would you like to go?" After they get three ideas for each category, they will try to narrow it down. For example, "You said you would wish to go to Disney World, Hawaii, or the Bahamas. If you could only go to one of those places, which would it be?"
By the way, have you heard of Give Kids the World Village? It's where all the wish families stay when they visit Orlando. It is quite literally the happiest place on Earth. I really can't do it justice here, but it really is fantastic. Each family gets to stay for six nights in a two bedroom villa. They get a rental car and tickets to Disney, Universal, and SeaWorld. At the village there is a food court, an ice cream parlor (open for breakfast), nightly parties, a swimming pool, carnival rides, horse back riding, video games, fishing, character meet and greets, and literally anything a kid could wish for. Not only is everything handicap accessible, it's also 100% for families. I think your son would love it, because he could visit Disney, visit Universal (the have a Minion ride and you can meet them and the Despicable Me characters) go swimming, meet Mickey, and go on carnival rides (there is a carousal, a flying ride, and a train). Of course, you have many, many other options, but I think he would have a great time there. If you have time, go online and find some videos. Also, if you don't think he will like it, go on the MAW America and some of the local chapters' websites. They usually list past wishes, and you and your son can go through some of the past wishes to get an idea.
Good luck!