@knockedup,
He does it because you let him.
Seriously.
Staying with him for your safety when you know he's violent and doesn't respect you -- regardless of how he is with your child -- is you giving him a license to keep doing this.
And he will.
Do read about Battered Woman Syndrome, okay? Not that you have been hit at all -- you may not have been -- but you might want to, honestly, see if you've got any of the symptoms.
Oh, and to the other members of A2K: I bet dollars to doughnuts that this will turn into a long discussion. So let us agree from the get-go:
the paramount most important thing here is the safety of this woman and her child.