1. Divulge no sources. None. Do not let on at all as to how you obtained your evidence.
2. Say very little. Give no specifics. Just say something like "I am aware you have had a girlfriend even before we separated". Then say nothing and wait. Let him start talking. The more you sit and stare at him, the more he will feel the need to fill the emptiness and talk.
3. Give no ultimatums. But keep in mind, he who is most willing to walk away has the most power.
If you two are only separated at this point and you thought you were working on making things work, then in reality he was simply keeping you as a back up plan B.
How long do you want to put your life on hold so he can see if he is happier with someone else?