Here's a little bit of tough love from someone who has dealt with disability and expensive assistive devices her whole life and has strived to live as normal a life as possible...
Your son is going to be living with these issues for the rest of his life so you can either deal with them now and let him enjoy the benefits of his assistive device or deal with them later with him having missed out on a lot of important family activities.
Is there a reason why you can't fashion a hat that doesn't interfere with his hearing aid? You probably won't find anything in the store, but you should be able to alter a hat...cut a hole or something. He will need a hat the rest of his life and shouldn't have to choose sun protection vs. hearing.
I would recommend stopping by Guest Services to get a Guest Assistance Card (GAC). This way you can sit in an area of the theater attractions that provides the best acoustics for your son. If you find you need to sit near the front, don't be shy about letting CMs know your needs and request to be put in the que that allows for the optimal seating for your son. You can also use the card to request a quieter area of a restaurant. The card might not be necessary, but it can be used to show inept CMs and rude guests that your son has a NEED vs. a WANT. It's also polite to use the GAC system because it makes it easier on the CMs who you encounter. (Keep your GAC card from year to year to make it easier for the Guest Relaitons CM to issue your family a new card.)
I don't see how you can justify not letting your son hear the entire day because you as a parent might forget to take his hearing aid off at the necessary times. Removing the hearing aid before getting near water should be as reflexive as not letting your kids run into moving traffic or having access to knives. If your kids have stayed safe for five years under your supervision, you have the parenting skills to not get the hearing aid wet...if you make it a priority.
Again, I understand your concern since your son is so young and the Disney crowds can be chaotic...but I honestly don't see any reason to worry if you just remember that your good parenting skills don't stop once you enter the gates.
I apologize in advance that this post might come off as harsh. Making the transition to leading a fulfilling life with a disability often requires a steep learning curve of assertiveness and creativity.
I'd love to hear how things went after you get back from your trip!
