He's doing this because you're letting him. You should be able to walk away and know he will not do this, as well as know he won't just because you say not to. Consequences. Discipline. It isn't mutually exclusive of paying attention to him. You can do both. Do NOT let him get away with hurting the baby EVER without a consequence. If there is no consequence worth avoiding, there will be no reason for him to stop. He gets praise and hugs for helping, negative consequence for trying to hurt. He'll learn.