I'm generally not ok with physically punishing children... But in this case you're dealing with a kid who is using violence against someone else, and I think the punishment fits the crime. He's gotta learn that if you go around hitting people you're gonna get hit back... I guarantee he will stop when he figures out that he will feel pain each time he inflicts pain on someone else. It's one of those things where a severe punishment is warranted... if this behavior is not stopped NOW, while he's still young, it's only going to get worse and worse. Eventually he will either seriously hurt someone or someone will kill him.

However, I'd advise against using the belt because the kid chewed with his mouth open or doesn't want to go to bed... The belt should be reserved for only the most serious/severe circumstances where the child has done something awful and knows better. Also, never do it if you're angry. If it's used over and over not only does it lose its effect but you will just make the child spiteful and rebellious (trust me, I know that from first-hand experience lol).