What to do when a teenager is being pulled down by his girlfriend?

My son is almost 18 and has been friends with a girl for about a year and a half. She is a cute little thing, but comes with a lot of baggage and from a broken family. I didn't mind the friendship, but my son is on the phone with her for many, many hours every afternoon and she constantly gives him her personal items so she has an attached excuse to gain access to him. He is a senior in high school this year and I'm seeing his grades fall, he looks like the worlds saddest person, his talk has become defensive and mumbling. We have pretty strict rules, but is starting to break them. He's already made the committment to go to prom with her, but I'm not comfortable. I'm hoping that after graduation, when he goes to college, things will fizzle. He's not a bad person, it's just that he is depressing and withdrawn. He goes to church with us on Sunday's, however, he doesn't want to participate in any other activity with us or his siblings. Any suggestions at this point?