While activities for little princesses at WDW abound, you have to look a bit harder to find something tailored especially to little boys. Not to worry though, Disney has you covered.
You don't say when your visit will be taking place, but at the end of June,
The Pirates League will debut inside the Magic Kingdom. It seems like it will be sort of a masculine counterpart to the Bibbidi Bobbidi Boutique. Located in Adventureland, the Pirates League is a place where kids of all ages can go to earn their sea legs. Three different packages are available and each will effectively transform landlubbers into pirates in no time.
Your son might also like a pirate adventure cruise. These cruises (for kids aged 4-12) set sail from a variety of resort hotels. Each of them last approximately two hours and include lunch and a treasure hunt. More information on specific options can be made available by calling 407-WDW-DINE.
Hopefully one of these options will suit your son and will be the perfect counterpart to the perfectly princess tea party you've got planned for your daughter.