He is asserting his dominance over you in a physical way. 1/ this is totally unacceptable behaviour, showing no respect for your person. 2/this may well be the thin end of the wedge so to speak. He has very violent tendencies, that are at the moment suppressed, but could easily get worse causing you serious harm. 3/ Gradually fade him out, decreasing your time together until you break up completely. 4/ tell him now, that it is over, he's unstable. Please ditch him.