So i have recently been on the other side of this- where my partner blacked out twice and cheated on me twice. He didn't tell me about the first time, but came clean the second time because he realized it was a problem that he thought he got under control after the first time, and then realized that he didn't. Honestly, I think that not telling someone the first time is okay- I would have done the same thing. But the second time, I think you owe it to him to be honest about who you are and where you're at in your struggle with alcohol just my perspective