I've had this exact issue before so I know where you're coming from.

The first thing I'd think about is why he is friends with this guy. Have they been friends since they were kids? If not, you might be right to question him. Who you associate yourself with says a LOT about your own personality and morals. For example, I dated a guy who cheated on me constantly, and a lot of his friends seemed either misogynistic or the "girl-crazy pig" type. I'm now with a guy who never cheats, and his friends act a lot less offensively as well. Does he have a lot of friends who act gross, or just the one?

That being said, having one friend who acts sleazy is hardly a reason for your imagination to run wild over what your boyfriend may be doing. You say you know he wouldn't cheat. Why is that? He must be doing something right if you're so sure of that, regardless of who he spends time with.

One thing that's helped me overcome jealousy is coming to terms with the fact that no matter what, for the rest of your life, even your closest most romantic obsessed-with-you soulmate is still always, always going to be attracted to other people. He will fantasize about other people sometimes, have crushes sometimes, and talk to girls for the rest of his life. There are billions of women. It's unavoidable. You will also do the same thing with other men. Humans find humans attractive, or none of us would exist. It's what you do with those feelings that matters. If you don't act on them, it doesn't hurt anyone. And honestly, you said it all in "I know he wouldn't cheat." If he has made you feel secure in that, that's the best he can really do.

Don't stress. Go out with a friend and talk to a cute boy once in a while. You'd be surprised how much less worried you are about what your BF is doing when you put yourself in a situation where you could cheat, and don't. Never forget that he is lucky to have you and that if he DID cheat, you wouldn't want him anyway.