The problem is not entirely THAT you cheated, but WHY you cheated. Obviously your marriage is not solid. You have two choices: either get to the bottom of what is wrong with your marriage and fix it
Make plans to end the marriage and move on.
I would not tell him about the indiscretion you had with that other guy because it will infuriate him and reading between the lines, you will have holy hell to pay. So, you need to reach deep down inside yourself and find out what you want. Do you want to stay with him or end it? Pick one and follow through with a vengeance. So, you either fight like hell to save your marriage or you give up and dismantle your life with him and move on without him. Those are your two choices.
Should you ever tell him? Well, according to Bill Clinton, you did nothing wrong!!! Laughing my @ss off!!!!