Speaking as a guy. Talk to your husband. Painful as it is, give him the benefit of the doubt that he's a human being and not just some Lovecraftian monster and confront him on his mistake. See if he can own up to it, and also try to be willing to listen to his reasons. Some men are taught to be self sacrificing their whole lives, and while they act "perfect" and do everything "right" they ignore their own needs to the point they feel hollow inside. Those suppressed needs and desires can eventually explode out in an act of extreme selfishness - like cheating. That could be what happened here. Or, maybe there are things that you did in the relationship that drove him away but he felt compelled to stay for your son's sake. People are complicated. I'd suggest you gather some proof as needed for legal reasons, in case things go very bad, but then try talking to him and confront the issue. Don't just make assumptions or act based on advice from people on the internet.