Your description of your husband fits someone who is immature. He is not prepared to take on the caring and responsibility of being an adult spouse and parent.
From what you have described, he is probably good at dating and early-stage relationships, but he does not know how to stay in love for the long-haul. You are not the only one who will hurt from his behavior. Your son will miss out on not having his own father living in the house with him, perhaps even growing up thinking his father left because of him. There are many other difficulties associated with growing up with divorced parents.
Even if your husband has not been physical with the other woman, you could still call this at the very least an emotional affair and he is demonstrating a problem in his personal character.
You might try getting him to see a marriage counselor with you, but even if he agrees, he can still single-handedly sabotage the therapy and your marriage. I am sorry to have to give you this bad news, but I suspect you already knew.
