M.O.
He'll grow out of it, I swear.
For right now, though, if he's hitting with a toy, take the toy away. Put it up high where he can see it but not reach it. And keep saying, over and over and over again, "gentle" or "touch nicely" (tell him what TO do, not what NOT to do). Ask the sitter to "put his toys in time out" every time he hits with them.