Why is my son acting like this?

I have a 14 year old son. He plays lacrosse, soccer, basketball, ice hockey and he runs. He is a good kid in school he always makes good friends, has nice things to say, gets straight A's and he has a nice sense of humor. He is open about his feelings and tells me when he's upset. He comfortable talking about safe sex, privet part problems and ext. He is always doing things he's told. He always helps out his 3 year old sister and is very close to his 17 year old brother he is adored by many of the little girls at his school (they love his "Justin Bieber hair" lol) . As in the last 9 months he has been acting quite weirdly. He doesn't help out anymore, he has became less close to family and friends, most of his tests are C and B's sometimes D's!! He hates himself and blames everything on himself and he's more sensitive and he doesn't want to talk about anything anymore. I think it's his girlfriend. Shes always cursing which makes him curse. She always says messed things about other people and that girl spells DRAMA all around! How can I get him to understand she is bas for him??