Here's a slight twist on the suggestions that have already been offered...
If MK isn't your first night at WDW, then try and take the little one over to the Poly for a "preview" of Wishes. He'll get to see the Electrical Water Parade and Wishes from a safe distance. I would still have him wear the earplugs at the Poly so he is used to them.
You're not alone with this problem, as I think most youngsters have problems with loud noises. Our DD is 7 and still doesn't like loud noises, weather they be from fireworks or even an announcer at an attraction. This is wear earplugs will come in handy. We'll be bringing them along for DD7, but I'm not quite sure what we'll do with DD1 yet. She's still in that "everything goes in the mouth" stage, so I can see her taking the earplugs and popping them in her mouth.