What do you mean "what do you do?" You already did it. Done. You broke up with him. Don't answer his calls. Instead, use your telephonic communication device to order a pizza. Then don't think about him. I know, easier said than done. But here's a game: replace his name with a word that is negative and unattractive. For instance, if his name is "Frank," don't think "I wonder what Frank is doing?" Think, "I wonder what war crimes is doing?" Or "I wonder what rectal vomit is doing?" See? Simple. It's psychological-ish.
Why would you be jealous if the jackass crawls back to his ex? That's her problem, not yours. That's a really adolescent manipulation move. Boring. Let him haunt this poor woman. You have your self-respect, personal happiness, oh yeah, and your son to worry about.
Dump him with extreme prejudice. Block his number. Change your locks. Delete all the pictures you have of him on your computer. Dig a moat around your heart and fill it with rattlesnakes. Get a babysitter. Go out with friends. Drink grog and make merry. Enjoy a cartoon movie with your love critter sans psychopath. Continue not to talk to him.
You might not know it, but you grew a spine when you broke up with him. Have faith in your instinct and celebrate your decision to kick him out of your life. Congratulations. Now is no time to go all wobbly: the two of you aren't dating anymore. So that's that. Good luck with your next bro-friend. From what little you've told me, he's not worth your time, or your sons time. Cut your loses. Please.
Relationships have two sides, like grilled cheese sandwiches. He has a perspective, too. There are reasons other than him being pure evil for his pulling a switcheroo from nice guy to jerk. For instance, his mother could not have breast fed him, or breast fed him too much. Once upon a time, he might have been a real prince. In the future, he could end up running a sustainable farm that supplies food for third world refugee camps. Who is to say? He is not part of your story anymore. If he wants to write me for advice, I will give it to him.
But this advice is for you, since you had the preternatural wisdom to write into me here at Guyspeak.
You dumped him. Good. Keep him dumped.