C,
You have received some great information so far. You are not married, you are out of the house, the kids are not his, so he couldn't fight for custody. You my friend, are blessed. I know you can't see that now and are looking for that one sure fire way to get him sober and live a happly life. It just doesn't exist with an active addict.
Why do you think that is acceptable to tell you he cheats, then tells you why and then the girl friend calls you. Yet you are still looking into his depression and worried about what he will say to you regarding his parents. Who cares!!
You need to cut ties, block him everywhere, phone email, all social media. Get into a support group and educate yourself why this is just wrong!! You sound like an amazing mom who has spent way to long with a self centered addict. Start directing your attention to your children, they deserve it. Hugs, and keep posting.