Stephen has decided that the "fist bump" is far cooler than giving high fives. All you have to do is ask him to give a fist bump, and he'll do it! Sometimes he'll just keep doing it and doing it, though... obviously, he likes this "big boy" greeting quite a bit!
It only took a few times demonstrating this for him to get it. Erik and I have both noticed that it doesn't take Stephen QUITE as long to figure out how to imitate motor skills as it used to. Hopefully this means that he is getting better at his motor planning!