It depends on your kid.
The Magic Kingdom is a definite, because it has the most that everyone can ride. After that it's a tough call.
Epcot doesn't have a lot in the way of "attractions." It does have live entertainment throughout the day and hands on stuff in Innoventions.
Hollywood Studios is mostly shows. It does have some key attractions, but if your son is too short or too scared to ride the Rockin Roller coaster and the Tower of Terror, then you may run out of things to do quickly. It does have a Star Wars section with a Jedi Training Academy that you might be able to sign him up for (slots go quickly)
The Animal Kingdom is mostly animals. It does have quite a few things to do, but it really depends on your interests.