My boyfriend is flirt, please help!!?

My bf used my laptop to log in his face book and the website just automatically remember his password so I can read everything from his messenger. I found out he is texting with a girl in this few days, they text a lot from around late night till 5 or 6am. I read it all, It is good when my boyfriend admit with the girl that he already have a girlfriend and he loves me. However, they still flirt each other a lot, i mean like a lot, seriouly. they made jokes about sex that they want sex from each other and in the future if my boyfriend is single he maybe be together with her. Or like he told the girl that she was so cute, beautiful and funny. It was only two days and they text like they've known each orher long time ago. Actually my bf is a BIG FLIRT but when i talk with him that I dont like, he just said he want to be friendly with all people. But text the girl like that is much more than being friendly :((. Normally, he is good bf, he cares about me and show me his love every single day from the beginning. Before he ever cried when I said want to break up. He come to me even he is so tired just because I need him. I didnt tell him that i already know everything yet. What should I do now?. should talk to him?