I can't give advice about forgiving... all I can tell you is that from my own experience, what I thought was just once, or was just online, or was just kissing... really was much much more. It was so painful to find out little bits of truth at a time - extended over a few months. I wanted so much to believe the "it was only...." but it was more. I don't want to suggest you become the cynic I've become, but try to get him to really open up and help him to feel safe telling you the whole truth. You don't want to find out more later. At least for me, I'd rather know everything there is to know. Good luck!