My son cannot avoid serious relationships!?

I have a late teen son who constantly gets to emotionally involved with a girl. If he goes out with a girl twice, he thinks he is in love. Then when they split, he can be miserable to be around, depressed and has been suicidal in the recent past. I worry that he defines his self worth as a person by his dating relationships with girls. He also keeps lowering his dating standards, I am assuming this is so that he can be assured they will not leave him. He used to date well mannered Christian, athletic, ambitious, polite girls, he is now dating a morbidly overweight, non-Christian, unattractive, unambitious girl that has on multiple times expressed interest in doing drugs with my son! ( He refused!) HELP!!!! What can I do other then talk to him and try to explain to him this is not a good idea for him emotionally and that he really needs to focus on finding himself and developing his future. I try to get him to just hang out with frinds and focus on school, but no such luck. He used to be involved with school sports and dropped out, his grades have dropped, he never hangs out with his friends, he also used to play music and never does that either. Hi head is so far this girls a$$ his life revolves around her. Our entire network of family and friends are at a loss as to why he is with this girl or why he gets so wrapped up at his young age with ANY girl!