Ask yourself why you feel this way? Why don't you trust your boyfriend?
Implicit in your letter is that your boyfriend should not be engaging in extracurricular activities — at least in your mind. If you haven't already done so, have an explicit conversation in which you go over what the rules and expectations for the coming months are. Perhaps he assumed they were different.
If he agrees — and says nothing is happening — you have to trust him.
If you can't trust him, the relationship's not worth having. End it. It's not like you're not surrounded by beautiful Frenchman at every corner. Oy vey, if I were a 40-year-old youngin' again …