Hi Elizabeth!
Magic Kingdom to me is the best theme park for children, especially pre-school aged children.
From It's a Small World, to Fantasyland, to the Transit Authority PeopleMover, Magic Kingdom tailors to the little one's, and certainly provides an enjoyable experience for both parent and child.
To see what's all available for your son at Magic Kingdom, click this link here to go to the "Attractions" page.
Then, to the left, choose "Location: Theme Parks & Water Parks" and place a check mark next to "Magic Kingdom."
Next, choose "Recommended Ages" a little further down, and place a check mark next to "Pre-Schoolers."
This will now filter all attractions at Magic Kingdom to children of pre-school age.
Good Luck with all your Disney planning!