Hi Erin,
Thanks so much for bringing us your great question! I have a son who struggles with different sensory issues, so I completely understand your concern and am happy to help you figure out a plan to help your son feel comfortable!
The Many Adventures of Winnie the Pooh has a queue that is pretty open and exposed. While parts of the queue are covered and even indoors, it's all pretty open to the outdoors and I think he would still hear the Happily Ever After fireworks. The ride itself is indoors and I don't believe you'd be able to hear the noise while on the ride, but it lasts for just a little over three minutes while Happily Ever After lasts about 18 minutes. Most of the queue for Under the Sea - Journey of the Little Mermaid is indoors, and because you would be experiencing this attraction during the nighttime celebration, I would expect the line to be rather short, meaning you should just walk right through the portion of the queue that's outdoors. The ride itself is also indoors, and this one lasts a little over six minutes. If you timed it correctly, I think you could be able to avoid all of Happily Ever After on this attraction, however there is a chance that you'd be done before the fireworks are over.
Have you thought about trying
Mickey's PhilharMagic during this time? With an indoor waiting area and a 12 minute show, you should miss the entire fireworks show happening outside. It is a dark theater and there are some 3D surprises during it, but it's one of my family's favorite shows. Another really safe bet would be just around the corner from Fantasyland at The Hall of Presidents. That show lasts for over 21 minutes and is indoors as well. Whatever you decide to do, definitely feel free to ask for help from a Cast Member if you need anything to help your son! I hope you and your family have a truly magical vacation, Erin!
Missy