Did I make a mistake by staying?

Your best bet is to leave for good. Your son is learning how to treat a woman and he's learning to be abusive. No mother wants that for their kid. He's not happy, he's a kid. He doesn't know what happiness is. It may be jarring for him but in the long run it will be better for him. Just like when you have to give him medicine he doesn't like. Mommy knows best and you shouldn't second guess yourself.