Hey there! (((massive hugs)))
If you are going to confront him, you need to channel your inner bitch. Stand up tall and show as little weakness as possible. DO NOT TELL HIM how you got your evidence. You may need to go back to the well, as it were. DO NOT LET HIM derail the conversation by dragging in explanations, whys, etc. Say, We'll talk about that later, right now we're talking about XXX. Imply that you know far more than you do and keep bringing the conversation back to his cheating. "You and I both know that you did this, you need to come clean to me. You need to tell me the truth. You were not as clever as you thought, I know what you've been doing. You need to tell me the truth. It's none of your business how I know, the point is I know. You need to tell me the details." Keep the hammer fully down upon him, do not let him put it on you.
I made a mistake No, you made a DECISION to cheat.
It isn't what you think Yes, it's exactly what I think. You cheated and were trying to get laid.
I don't have to answer your questions If you want to sleep in this house tonight, yes, you DO need to tell me the truth.
And remember. He is a liar. Liars lie. So don't believe that he was "fooling around." He was looking to get laid and probably did. (((hugs)))