A lot! You will find that the vast majority of all rides and attractions in Disney are open to even the youngest of visitors. My own daughters have been going since they were newborns and have been able to experience almost everything even at those young ages.
I surveyed my 4 year old and her favorites in Animal Kingdom are: Kilimanjaro Safaris, The Boneyard, Jammin' Jungle Parade, and the Kids Discovery Clubs.
In Magic Kingdom, her top picks are Tom Sawyer Island, Buzz Lightyear's Space Ranger Spin, Goofy's Barnstormer, it's a small world, and the Tomorrowland Transit Authority PeopleMover.
The
attractions page on the Disney World website will be a great resource for you. Not only can you view all of the offerings in each of the parks, but you can also sort by height requirement. If your son is tall, a lot of 'big kid' rides open up for him!
Have a great vacation!