PTSD turns prince charming to rageful monster?

I have been dating/living with my bf for a few months now. It has truly been more perfect than I could ever image. He has always been a true gentleman to me and treats me kinder than anyone ever has. He saved me from alcoholism and we brought eachother closer to God. Recently when we bicker over small things it somehow blows up out of porportion. I can tell that he is filled with frustration and he feels like he can't get through to me. Not an excuse but he suffers from PTSD from the war and witnessing a horrific accident. At 1st he pushed me up against the wall and spoke really close to my face, this was not like him. I didn't mention anything about it after, i just brushed it off. Than a week later he got so frustrated with me bc he thought i wasnt listening to his points in a disagreement he grabbed me by my ears and shook my head while talking really close to my face again. I just started crying which made him MORE frustrated, he then grabbed me by my throat and choked me while he was in my face saying "Shut up! Shut up and listen!" I could tell by looking in his eyes that something had come over him. He was always so gentle and kind, how could he turn this way? When we made up I told him that this could never happen again. He was disgusted by his actions and showed alot of remorse. The next week we got into a disagreement. I am never the type to back down bc although we had the altercation before i was not scared he was actually going to hurt me. I was wrong. I told him to shut up and he jumped ontop of me and began shoving my head into the bed, punched me three times in the same spot in my arm leaving a huge weld. He hit me in my temple, it was so hard i heard ringing for a half an hour. In that moment i was actually scared for my life. It is like he turns into a completley different person. It hurts so much. I can tell that he cant control this. We both agreed he needs help. I dont want to live being petrified to push someone over the edge. How can I help him?