What should I do? Leave him or stay with him?

My boyfriend of seven months has started treating me different. I lived in Oklahoma where I met him, and we dated 3 months there. Then I moved to Tennessee, and for the first few months he treated me the same way. Now he treats me completely different, he doesn't act like he loves me anymore, and he flirts with my best friend constantly. This used to bother me to no end, but I know that now, she doesn't like him ANYMORE. The rest of it still hurts though. I really have no idea what to do with this guy, I'm really attached to him, and I love him alot.