Relationship Advice.....?

[Rosssieeee]

Hi!
I am not married nor divorce but I infer people in this category will make better answers.

My boyfriend and I are 17 years old and we have been in a relationship for 5months. We have each others passwords because we both believe we have dont have anything to hide.
My boyfriend moved November 2011 to my current town. He has family where I live and decided to move for better opportunities (education/sports).
I recently ( not even 3 hours ago) read his DM's on Twitter from a girl that went to school with from his old town/state and how she misses him and he is coming home Christmas break and he was going to surprise her and she wants him to be her christmas present and how it will be the best christmas she ever had and all that crap!! Also how for New Years i how they would bring in the new year like old times. How should I go about approaching him about this?

I have his password for God-sake, he is my first boyfriend. Are boys really that simple minded?