Hi, Shari! When my oldest son was 3, 4 and 5 years old, he hated loud noises too. It was never appropriate to ask a boy of his age to man up, so we put the ball in our parental court and smartened up.
We purchased some squishy earplugs for him to wear at louder attractions, such as fireworks. They worked like a dream during Wishes and Illuminations. We put them in for Fantasmic and he took them out because he didn't find the noises that alarming. Go figure! At that time, we were with some friends who had two young girls that also had a fear of fireworks. They were so engaged with the story that the pyrotechnics didn't faze them either.
You'll see all sorts of recommendations for when you should get in line for Fantasmic. Unless you're there during a very crowded week, I think arriving 40 minutes prior to the show does the trick. If there are two shows of Fantasmic that night, go to the second one (it will be less crowded.) You can also take advantage of the
Fantasmic dining package if it's in line with your travel plans.