OP
Try and stay calm when you talk to him. Keep it really simple.
Watch his face and his body language. Watch his eyes.
You know him, we don't. Take your time and sit with him, next to him even.
You don't want to be seen to be attcking him, he'll just clam up. He has to let you have your say but if I were you, I'd ask a question and then wait until he answers you before you carry on.
Hard to do but.
The chances are that you won't get the whole truth when you first talk. He may give you the basics if there has been something going on, and perhaps, over time, he'll tell you it all.
If he has been unfaithful, he hasn't really thought about what he was doing, otherwise he would've hidden the evidence far better than stuffing it in his bag.
Maybe it was a one off? Maybe it wasn't.
Don't let him control the situation.
He'll most likely try to make you think it's you or that you're imagining it. He'll be gutted that he's been caought. Assuming there is something to be caught for.
Just take your time. This is your life and your marriage here, not ours.
Good luck x