I know everybody wants things to be the best they can be for the kids. But your boys are old enough to know the truth, and I don't think that lying to them about a "mutual" separation or temporary thing is right or fair. You don't have to get into details, but a simple "your father has been having an affair and has decided to move out" is sufficient.
Meanwhile see an attorney ASAP and file an order for temporary support.
I totally agree on not having him on Mother's Day. He might as well start feeling the consequences of his actions immediately. And nothing wrong with calmly slipping in a couple of truths, such as "you know you're going to regret losing a good woman like me" and "just remember, if she'll cheat WITH you, she'll cheat ON you". He won't admit those things but they'll stay stuck in his head, trust me.
Now do something fun with your boys tomorrow. Go see the new Marvel movie. Put on a brave face and let them know that the three of you are still a family and that you're all going to be fine. They will need you to be their rock.
