Hi Kelsey! I am so sorry for the experience your son must have been through with a tornado. I hope some Disney magic will help ease his pain and allow him to move ahead.
It is difficult to know exactly what will be deemed a loud noise or unexpected noise for your son. I suggest you take a hard look at all of the
Disney attractions in all four of the parks. First look to see what rides your son can go on as there are some height restrictions for many of the thrill rides. Then read the descriptions of the rest. Look strongly at the Fantasyland attractions at the Magic Kingdom as most of those will be excellent choices for your son. Peter Pan's Flight and Its A Small World are favorites as well as Winnie the Pooh and of course Dumbo. Philharmagic and Pirates of the Caribbean will have loud noises.
In Epcot consider enjoying Living With the Land, Under the Sea, Imagination, and of course the great Character Meet and Greets. Take time to enjoy Rafiki's Planet Watch in the Animal Kingdom park. This is a great place for your son to get close to animals and even learn how to brush some of the resident goats and sheep. The Safari ride is a must and I think he might really enjoy the Lion King Show which is full of music. Hollywood Studios is a little tougher park to recommend to little ones who do not handle noises well. The great Toy Story Mania is so much fun but it is loud with a lot of shooting pop guns at targets. There are some Star Wars things he can enjoy and of course the Disney Junior show. Avoid the Muppets as that has a lot of loud sound effects.
Fireworks might be avoided but the parades are wonderful. I suggest you bring some children's headphones with you and use them if you feel something is getting overwhelming to him. We use them for our granddaughter when she goes to sporting events and it really calms her down. Hope all this helps and I am sure you will have a magical vacation!