Karma is a Heartless Woman
Agreed with all of the above. This guy is just a needy nobody who needs to leech off of other people in order to be successful. Eventually, his neediness will be the demise of him.
Although I do strongly agree with everything above, I would like to throw in another input. The reason he is taking women from you is because he believes himself superior to you. He knows that because of his looks, he can pull women off of you. He also knows that from past actions, you won't do anything bad to him, because he knows he's an alpha male, and the betas don't try to keep from the alpha what the alpha wants.
The best way to deal with this is to not try to maintain the woman's attention. Your best option is to actually shift your attention to your roommate. Talk to him, get him really conversing. Let his true colors come out in front of her while dropping subtle DHV's about yourself and DLV's about him. This has the effect of showing her that although your roommate thinks he's hot stuff, you are the actual alpha male, and you are superior to him in every which way.
On a joking side, If you really want to get even with him, start giving him bad advice. Give him stupid rookie mistakes as if they were expert options, and give him some really terrible conversation starters, negs, and openers for him to work with. It's almost like spitting in your food after you have been sick and after your roommate keeps eating it without your permission, and then finally telling him after he does it again and is already throwing up all over the place.
D1ck move on your part, but what goes around comes around.
"There is no better than adversity. Every defeat, every heartbreak, every loss, contains its own seed, its own lesson on how to improve your performance the next time." - Malcolm X