Personality Disorders Support Group
Personality disorders form a class of mental disorders that are characterized by long-lasting rigid patterns of thought and behaviour. Personality disorders are seen by the American Psychiatric Association as an enduring pattern of inner experience and behavior that deviates markedly from the expectations of the culture of the individual who exhibits it.

Second, it's a good thing you haven't done the usual and dumped him impulsively as a friend. You obviously recognize that it's not health to just dump friends for no real reason.
Third, you might want to consider if he is really the one for you. If he is going really far with girls, he might have other interest and be more advanced than you would really want to get involved in.
I'ts kind of like once a guy or girl has experienced something exciting and new, they will more than likely want to continue that something exciting and new regardless of who they are with. I refer to it as a water switch. My water switch was turned on when I was a kid, so I couldn't turn it off later on.
Also, don't be so hard on that slutty girl. I realize she is not someone you will like, but I have found people are often slutty because of things that occured to them when they were younger and it was out of their control-or at least that is what happened to me.
****I would really consider the "he might not be the one factor". There are so many cool guys out there. My husband and I bumped into each other as kids and teen without noticing each other until much later. When you do get in a serious relationship, you are going to want someone who put you first, or actually second-God first. Female friendships and serious relationships are two different things. I know I am not the world's most whatever, but to my husband, I am. That's the love factor that kicks in.