Simple gabbe, talk to the police. As intimidating as going to the police is, it is a must, and the officers are there to help you (if I didn't want to be a teacher, I'd probably be a cop). He assaulted you, and technically, that is a crime. Most people don't realize that if you punch a guy, that is assault (or possibly even battery or aggravated assault, depending on the situation). You CAN go to jail for that, or in the case of a minor, more likely a fine, community service, restraining order, etc; (Of course, technically, threatening to "kill someone" is also illegal, though few people report such things).
Speaking of restraining orders, if you truly believe this person or his dad is a genuine threat to you and you can prove it, a restraining order may be a good idea. This will make it illegal for them to come within a certain distance of you. Not sure about them on minors though, I've never seen such a case.
Anyway, I think you may have handled this a bit badly originally. When bullied, just ignore them. As tempting as it is to slip them a snappy comment, you'll just risk making things worse (see what happened?). In the future, just walk away. Yes, it sounds cowardly, but you'll be glad you did it. People can do very stupid things when aggravated (one case I recall was when a guy was killed with a knife because he stole a guy's hat).
