I think
@Desdinova's suggestion is a good one. Now getting him to actually move out is something else entirely.
Have a lawyer draw up the separation documents and serve him at work. He will know you are serious. On the day of service change the locks on your house. This man is unlikely to move out of the family home on a volunteer basis in my opinion. I'm an ask forgiveness rather than permission girl on this issue.
And it's not something your son will understand. You see, your husband knows what a good mother you are. In fact he's betting on it. He's betting you'll put up with his behavior out of fear of hurting your son. That you'll sacrifice yourself in the marriage no matter how rotten his behavior. Children are much more resilient than we give them credit for. Don't let your fears suppress you.
Des is correct. This husband has no idea how good he's got it. Really good women are hard to find, even for wealthy men (who have to deal constantly with gold diggers) who may not realize how vacuous women can be.
A few weeks ago I met a man who is 35, rich, successful, hot, witty, and smart. Type of man most any woman would find very desirable. The man resides on the water. In Maui. He is never married and was lamenting the market. Pretty, hot sexy women are EASY to find, he said. Sex is EASY to get, he said. He can't find a good girl with a good head on her shoulders. He wants a woman who "can make me a better man" as he put it. A man like that would treasure a woman like the OP.
Des is right. Let him depend fully on the mistress for a while. That will help him see with new eyes.