He most likely won't 'outgrow' a standard 20" BMX bike anytime soon, but that depends partly on the top tube size. If it's somewhere around 19 inches that should be good, and it'll be fine if he grows another foot and a half or so. After that if he still likes riding, he'll want a better bike.

Pedaling sitting down all the time is a bad habit, he will try to sit down and go over jumps and end up smashing his balls. The 24" bike will be hard for him to maneuver around, it'll be almost impossible to manual and hard to wheelie.

I echo the recommendations to get the 20".