Hi Holly!
Welcome to the Disney Parks Moms Panel! I have a Mouseketeer who has medical challenges, too. I completely understand the importance of steering clear of bacteria and planning ahead as much as possible. One entire suitcase of ours is usually filled with medical equipment to help keep my son's lungs clear in case he does pick up an unwanted "souvenir." It's my pleasure to share ideas with you on how I keep my little one only visiting attractions and Disney Characters, not the hospital!
Any of the extra planning required to make sure your little guy is healthy will be worth it because your entire family is going to have so much fun at Walt Disney World Resort! One resource that might help you plan which attractions have water elements and which don't is the
Resource for Guests with Cognitive Disabilities. If you look at pages 21 - 38, you'll find a sensory checklist of popular attractions at each of the four theme parks. There is a separate column for "wet." If an attraction has a water element to it, the column for wet will be checked! (Please note, attractions are always being added throughout Walt Disney World Resort, so you may not find every attraction listed.)
At the entrance of each attraction, there will be a Cast Member ready to answer any questions Guests might have. I highly recommend speaking with him/her about how important it is for your son to avoid water and mist. They'll be able to tell you if the attraction has either and what to expect!
One of the MANY reasons we love vacationing at Walt Disney World Resort is because of how super clean and tidy it is! With that being said, we always carry hand sanitizers and wipes with us for extra protection against germs. I also bring medical masks for my son to wear while in enclosed spaces surrounded by people.
Beth