Considering you've already experienced Animal Kingdom and Magic Kingdom, Julia, I'd recommend Epcot and Hollywood Studios for this visit. I know full well that many people consider Epcot to be an "adult" park, but I disagree wholeheartedly. There are plenty of attractions at Epcot the younger crowd will enjoy, including:
Soarin',
Turtle Talk with Crush, and
The Seas with Nemo and Friends. My five-year-old son also enjoys visiting the
Kidcot Stations all around the World Showcase. Oh! And then there's
Innoventions. Really. There are a plethora of discoveries to be made at Epcot.
At Hollywood Studios, your son will likely fall in love with
Toy Story Mania. After our first visit there my son came home asking if we could give him that attraction for his birthday (as in, build a replica in our backyard!). If your son is into cars, you should check out
Lights, Motors, Action! And don't leave without seeing Fantasmic!
If you take in the two parks you've yet to explore, you'll certainly come away wondering how on Earth Disney packs so much excitement in each and every one of their amazing theme parks. Have a great trip!