How do I talk to him?

I posted last week about if I was being too jealous about my SO and his girl friend. How he brings her up a lot in conversations, has a cute nickname for her, texts her at least a couple times a month, and told me that he would rather break up then stop being her friend if I ever asked him to stop talking to her (which I would never do), how they went on what seems like a date to me and didn't even tell me until a month later when he brought up how jealous the girls SO was being when they had gone out for the day together an left her SO behind..... There was more too but anyways now he is going back home for 2 weeks and that's where the girl lives. I never really got a chance to talk to him about this because I don't know how to go about it. He leaves in a month and it makes me really uncomfortable that she is out there and he might still have feelings for her. So how should I go about talking to him? Should I just come straight out with it right when he opens the door and say "are you still I love with her?" I don't know how to bring it up! Please give me advice on how to go about this