Reading Our Private Messeges

So I have been dating my boyfriend for almost 2 months and we dated 2 years ago and just got back together, just to be clear. He has a girl bestfriend and I am very uncomfortable with her. He asked me why I don't like her even though she doesn't live in the same state. I honestly don't like her because I think she has a crush on him and she's always calling him "kitten". She has his password to facebook and she's always reading our messeges which I am very uncomfortable with because she can screenshot the things we say and send them to a friend or post them. How do I tell him that I want him to change his password so that she won't be able to see our messeges?