Bf is on Craigslist

Posted earlier but can't find it. 
My BF of 6 years cheated on me a few times a while back but I stayed. I know, big mistake. 
Well our son was playing on his tablet and got into his email and I did snoop around which I know was wrong, and found his replying to casual encounters to other women. I'm so hurt and heart broken. I don't even know how to bring it up