Think about the world from a kid's point of view. There is this absolute plethora of fascinating things to do and see and experience. Many of them are new, and what isn't new is often engrossing in the extreme. Plus, you know, it's always awesome to feel like you got away with a little something by pushing on some boundaries.
That's where they are when bedtime rolls around. It is normal to have them push at the boundaries. My expeirence is that most go through stages of this repeatedly in their lives.
As for the ex boyfriend, I STRONGLY suggest you tell him that you will consider working it out with him only if he can let go of his ideas about how you parent your children. Tell him that you would be happy to go see a counselor WITH HIM, but that you feel no need to see one alone and what you're doing is working for you.
Or really, better yet, tell him that you think his discipline methods are cruel and you want no part of being in his life, thank you.
The belt teaches a child fear, but it does nothing to teach him confidence or self respect. And that is clear from his own behavior on the subject as well. He is too busy being that angry child and lashing out at you on the phone to be that self respecting adult. He can't see his own damage.
Don't take him back into your home as long as he's attached to the method that didn't work for him. If you love him and want to see if you can get a counselor to work on him, you can go to counseling with him and let the counselor poke at his broken and see if he changes. But chances are he won't. Most people aren't ready to see themselves in the mirror.