Here is my take on it (only my opinion). I was in a serious relationship for 1 and a half years with a girl. At first things were great and then....it started. I discovered she was up late at night talking to guys, going out with guys claiming they were just friends (we lived separate). I didn't suspect anything as I am very trusting but sadly I learned the hard way. Long story short, she cheated on me and I got my heart broken. You have every right to be suspicious due to his behaviour but try not to dwell on it as that can drive you crazy. There is not much you can do if one wants to stray all you can do is hope they don't do it and be faithful.