My son is 4 and it didn't hold his interest the entire way through, so I guess it would depend on your son's age and his interests. A lot of the humor seemed to be over his head and more aimed at the adults.
My kids and I loved it. Pixar is an exception to the "sequel-not-as-good-as-the-first" rule. We chose not to spend the extra money for 3-D, but I think it was worth the couple of hours of entertainment.