It is not a good strategy to hide that truth in him. Because, firstly, it will make you paranoid which is very mentally exhausting. Secondly, if he knows you already knew it and you did not react at all, he may assume it's alright to do those stuffs and may try it behind your back and will make sure he won't get caught. (Sexual Proclivity). Don't count much on the idea that he will never know you knew those stuffs. He probably suspects you knew it already.
So, the best way to approach this is to discuss it with him. You may know and understand him better after that. Just make a neutral stance with honest conversation why you are curious to know. Assuming your BF has a mutual understanding of talking openly and honestly to you, your relationship may florish than you would expect.
It surely sound optimistic, but that is what we always do, I think, when we care for someone/something. That is we hope for the best. Surely, we sometimes expect some unexpectancies, but it's good to hope for the best when dealing with something important. Cheers