Son depressed whenever with girlfriend?

My son is seeing a girl who seems to bring a spirit of gloom with her (if that makes sense). When he is with other friends or siblings, he is happy and laughing. Whenever he is with her, he comes home depressed and a bit rebellious. She also has some classes with him and his grades drop when he works with her. I've tried talking with him and telling him that when you are in love, you are happy and build each other up, not drag each other down. Any suggestions?