Advice Needed: Boyfriends roommate giving advances and hurt me while boyfriend was gone.?

My boyfriend lives with another roommate, and we're all in our early 20's. He warned me at the beginning of our relationship to stay away from his roommate, but it was because he enjoyed stealing (or hitting on) all of his friends girlfriends and treated woman badly (emotionally). He's also into been into martial arts for a very long time and likes to rough house. It's been a year now and while the roommate and me are on friendly terms and do talk (they share a living space, it's not possible to avoid), I still have kept my distance but he'd occasionally hit on me.

Well a week ago when I was hanging out at the house, his roommate was home and walked into my boyfriends bedroom to ask me to check something on the computer. While I was checking for him, he said something stupid and I jokingly shoved his shoulder. In turn he shoved me back and I pushed him away. The little game suddenly escalated when he grabbed me though and locked my arms behind me (he's in martial arts) and it really hurt. I told him to stop but he wouldn't and kept hurting me. He grabbed my hair and it hurt to the point my eyes were tearing up and I told him to stop, but he kept it up as if we were still playing around. He dragged me by the hair across the bed for a while before he twisted my limbs into a lock again while I continued to tell him off. Finally he let go suddenly and left me there, completely in shock at what had happened and sporting bruises that appareared later in the day.

I haven't told my boyfriend what happened. I'm scared of what he'll do and the house is actually owned by the roommate and they work together (the roommate got him the job). I'm scared he'll lose the place he lives and his job. What should I do?