Hi, Jessica! You are the best person to answer this question, but I can certainly give you my experiences, having taken all three of my girls at very young ages.
All three of my kids LOVED the stage show type of attractions at Walt Disney World! I have video somewhere of my middle daughter standing on my husband's knees, dancing her tiny behind off at the Voyage of the Little Mermaid. My youngest, who is now five, was a huge fan of the Festival of the Lion King, and by two, she was singing along with the songs in the show, dancing, and clapping as loudly as anyone around us.
Some children are a bit frightened, as the music may be louder than what they are used to, but I've never had that experience with my gang of pint sized princesses. The louder, the better - they'd just sing along!
If by some chance your little guy does get bored or decides the Walt Disney show tunes are not his thing, there are exits clearly marked in all of the theatres. Just remember, you usually exit on the opposite side of the theatre from where you went in.
I'd say try him with one of the shows, and if it doesn't work for him, you may not want to tempt fate and try another one. But, you really never know. He may not feel the vibe of the Lion King, but Nemo is totally up his alley!
Have a great vacation!