Chiming in on noise cancelling earmuffs; I don't have any kids but I've seen bunches of kids wearing them in the parks during fireworks shows. I've always assumed it was because kids' hearing is more sensitive than adults' and the loud sounds were physically painful for those kiddos, but maybe they'd work for loud sounds being just plain scary, too. Also seconding sitting toward the back; being further away from the action helped me when I was a kid and was afraid of everything (much older than your son on the trips I'm thinking of, but still), plus it makes it easier to duck out during the show if it just gets to be too much. It's an open-air theater, and you enter from the back and walk down as close to the stage as you want to sit. It's very easy to get up and step out if you're in the back rows.
