I’d look at the dad, and consider what his escalation options are if I refuse to deal with him and just talk to his son. Once I realize that the dad can’t really do anything here, I’d just keep talking to his son.
“Hey, your dad seems like kind of a prick. Is he just a prick to other people, or is he a prick to you too sometime? I bet he’s pretty mean to you when it’s just you and him, right? You’re getting pretty big, you know you really don’t have to take that. I once got into a fist-fight with my dad. He was a real prick. Tell you what, he started respecting me a lot more once he realized I was willing to give as well as take. Next time, just haul off and wail him in the face. Guarantee you won’t be disappointed.”