Hi Anne,
I have a son who is sensitive to loud noises too so I completely understand your situation. At the Magic Kingdom, I'd recommend at the end of Main Street near the Train Station. We've also watched from Frontierland. Its not a great view but its better than a child having a meltdown.
You may also want to consider a view from outside the Magic Kingdom. We love the
Pirates and Pals Fireworks Voyage. You could also try viewing them from a private boat on a
Specialty Cruise. The one at the Magic Kingdom isn't loud but I'd be careful of the one for IllumiNations at Epcot which may be too loud for your little guy. We like to plan a dinner at either the Polynesian or the Grand Floridian and then watch the fireworks from the beach.
For IllumiNations, try the bridge from Future World to the World Showcase. You'll miss having a great view of the Earth Globe but can still see some of the higher fireworks.
Fantasmic! at Disney's Hollywood Studios is a bit more tricky. Its in a bowl setting and in order for you to view the show you need to be in the amphitheater. However, that puts you closer to the loud noises. The speakers in the amphitheater are big and loud so it may be too much for your little guy. There are a few fireworks at the end but nothing like the ones for Wishes or IllumiNations so I don't know that its worth trying to find a spot to see them.
If your son will tolerate them, you may want to consider purchasing a pair of earphones that he can wear to cut down on the noise. I've seen lots of Guests wearing them and they really seem to help. Earplugs may help some but if he's super sensitive, it may not be enough.
Wishing you minnie magical memories :-) Allison