Bustercrown: Sorry to say,but you got some really, really bad advice from whichever Rav you spoke to. Frankly, he seems to know very little about dealing with marital issues, addiction, pornography, or all three.
Of course you need to tell him you know. As it stands now, he is trying to avoid getting caught, so he lies. You become upset by his lying when you know the truth, and he is likely angry at himself for lying to you, but he can’t see another option. Continuing like this is a recipe for disaster and added tension and animosity.