I think the only way out of this is to tell your husband what you found, for the simple reason that whatever you do not say verbally, you will most certainly express nonverbally. To say it another way, it will soon become evident that you despise him, because of what he did in his past. And the more you try to suppress this - the more those feelings are going to grow. (It's sort of like trying to bury a worm - it just will crawl around & pop up somewhere else, probably bigger than before).
He may have a story that shows that he truly turned & repented from that lifestyle. I don't know - but you need to T-A-L-K I-T O-U-T!! Imagine if the Lord treated YOU the way you are treating your husband - holding a massive grudge, but never telling why, and so on.
Bottom line here is that you will have to learn how to forgive him, but I very much doubt you can do this in secret, because there is little doubt you are wondering what ELSE he may have done that you have no knowledge of...