If you're HP fans, after you do Ollivander's and FJ, don't forget to stop in Hog's Head for a butterbeer. It is awesome (and non-alcoholic, so kids can drink it too). I prefer the frozen kind, but the regular is good too. The line is usually shorter inside Hog's Head than at the outdoor carts.
And you can hear Moaning Myrtle in the WWoHP restrooms, it's kind of funny.
Don't know if an 8-year-old can handle Dragon Challenge (or is tall enough), but at least walk through the beginning of the line to see the Weasley's car stuck in a tree. And don't forget to look for Hagrid's hut while you're in line for Hippogriff.
Throughout the day you can also watch the Frog Chorus and the Triwizard Spirit Rally, both are short fun shows and you can have your photo taken with the Hogwarts students afterwards.
The rest of the park is great too. The others have good suggestions for the rides and attractions you should see.
Just be sure you are prepared to get soaked if you do Ripsaw Falls or Bilgerat Barges. If you get a cold day, you may want to skip those. You can either pay for a dryer afterwards, bring an extra set of clothes, or just deal with it like I did. You also get wet (to varying degrees) on Jurassic Park. I only got a few drops, some people got a little more wet than that.
If your son is into superheroes, you might see what time they will have the Marvel Superhero parade, or when the heroes will be available for photos.
For dinner, be sure to check park hours. The restaurants may close or stop serving up to 30 minutes before the park closes, if they close at 6 on a day you're there, you may want to wait and eat dinner at one of the CityWalk restaurants instead.