I know it's hard when this guy is father of your son, but you might feel better if you spend as little time as possible even thinking about you ex. Next time your son comes to you with some hurtful or insulting message that his dad has planted in his brain for him to bring home to you, tell your son that you'ld rather not hear about it. Don't concern yourself too much with what your ex is thinking. Unless it is about your son, your ex is not part of your life anymore. And it doesn't sound like that is any big loss for you. He doesn't sound too great.