There is (rather loud) music that plays throughout WWoHP - it is the music from the movies. It is also the busiest area of the park, not to mention there are two roller coasters there (at least one of the makes people scream). Loud noises are not really avoidable.
The two attractions with the longest lines are the Ollivander's Wand Shop show and the Forbidden Journey ride. Do those first, if the lines are reasonable. They may get longer as the day goes on.
The first thing you might want to check out, is the line for the Ollivander's show. It is a re-enactment of the scene from the first movie, except they choose a member of the audience to be in Harry's role. The show is cute but short. Kids will like it, but you might not want to wait if the line is more than 20-30 minutes or so. There are
youtube videos of the show, if you want to check it out.
Also check the line for the Forbidden Journey ride. It is often shortest first thing in the morning or late afternoon/early evening. On a Wednesday hopefully crowds will be low. FJ does include a dragon, spiders, dementors, and a few other slightly scary things - also quite a bit of twisting and turning. There are a few loud noises on the ride. If you don't want to do the ride, there is also a "tour" line that just takes you through the Hogwarts castle, where you can listen to the talking portraits, see the Defense Against the Dark Arts classroom, and see Dumbeldore and Harry/Ron/Hermione. There is a youtube video of the ride, if you want to see what it is like.
The other two rides, Flight of the Hippogriff and Dragon Challenge, were really short lines when I was there a couple of weeks ago (5-10 minutes). Dragon Challenge requires riders to be 54" and is a bigger coaster (big drop, lots of loops, 55-60 mph), you might want to watch it first to determine if he is ready to ride it. Hippogriff is a more kid-friendly coaster.
You can also check on times for the Frog Chorus and the Triwizard Champions, two short outdoor shows that are fun to watch. After the show, you can have your picture taken with the actors, which he will like if he is a fan of the movies. Or take his picture with the conductor in front of the Hogwarts Express.
You can also walk through the stores - even if you aren't planning to buy anything, Honeydukes and Zonko's are fun to look in. Ollivander's/Owl Post is too, if the crowds aren't terrible. There are also a lot of fake storefronts that are fun.
If you are there for lunch time, you can eat in the Three Broomsticks - but you might want to check the menu first, and make sure he likes that sort of food. You should at least try some butterbeer (regular or frozen), if nothing else. Check the butterbeer carts, Three Broomsticks, and Hog's Head to see which has the shortest line.
If he likes Chamber of Secrets, don't forget to walk in the restroom to hear Moaning Myrtle.
