He is probably seeing your son as a love interest, especially if he's never been around other birds. He may do more of this as he gets more hormones. How does he behave around the chickens?
It's normal in the sense that that behavior is fairly common in drakes, most of them don't act like that that but there still are a lot that do.
I think you are right about treating him like an aggressive roo. Although I'm not too sure you will ever actually get through to him that way, it's...