Hello, Cedarmom!
My older son (aka Thing 1) also is Profoundly/Unilaterally Deaf (SNL, left ear), with above normal hearing in his right. The type of issues your son has are very similar to what he experienced on a regular basis when he was your kiddo's age - if you Google 'Audiological Recruitment Issues", you'll find some interesting write ups on the subject.
Thing 1 now uses a Phonak CROS system, but used a TransEar Bone Conduction system when we first took him to WDW. He's a WDW pro now - we've been to the House of the Mouse 5 times, including a 10 day stay last June (one of the busiest times of the year). I asked him for his opinion, because he knows what he likes and doesn't at this point (his first trip, he was about to turn 10, and this last trip was right before he turned 16).
Likes: Almost all of the indoor line queue areas are actually not too noisy at all. He doesn't remember a single one being too loud for him - and that's saying something.
Meh: It's a Small World - mainly because we got stuck on it for 15 minutes our first trip. He'll still cringe if you start singing the song to him. (In other words - *nothing* to do with the level of sound...)
Be Warned: Aerosmith's Rockin Roller Coaster and Expedition Everest both have loud parts during the ride - but as he put it: "Mom, it's SO WORTH IT!" I honestly think he gets so distracted by the ride itself that it outweighs the noisy part.
Be Warned, Mom Edition: I can't remember if BAHAs are as non-water-friendly as Thing 1's CROS system. We made sure to carry a couple of sandwich sized Ziplock bags with us as emergency dry kits - and also got very good at warning him to clap his hands over his ears if we were on a ride where you had to go through a curtain of water (Big Thunder Mountain Railroad).
Restaurants: He can't remember having problems with any of them. I would tell you that the acoustics in the Disney restaurants are much better than at many of the restaurants we have here in our city.
Fireworks: The one part of WDW he really doesn't appreciate. I can tell you, however, that you can usually escape *most* of the noise by ducking into a gift shop and getting to the furthest-back-corner. (Not that we would know anything about getting trapped at the exit of Mickey's Philharmagic because we lost track of the time and it ended *right* in the middle of Wishes... ouch.) He did pack noise canceling headphones last trip and just carried them in a sling pack - we didn't have to break them out once.
Hotel: Does your son bury his "good ear" in his pillow when he's exhausted like mine does? Shrieking fire alarms *have* failed to wake him up in the past. I usually give the front desk of our resort a head's up that we have a hearing impaired kiddo in the party just for fire safety reasons (same thing when we cruise with
DCL).
I think he's going to have a great time! Thing 1 *loves* going to Disney, and believe me, he wouldn't look forward to going if he didn't enjoy the experiences we have there.