May I chime in?
Thank you, I think I will.
Please don't be distressed with the drake. Of course you don't want your son attacked, nor you, nor anyone else. I understand that. I don't, however, believe it is right to get upset with an animal for being who they are, instinctively and by design. For some reason, your drake feels threatened or is being protective. It doesn't matter much to me whether or not your son provoked the duck; I'm not jumping on that bandwagon. If he did, you will soon know and deal accordingly.
(I am guessing your son is a youngster, verses a teenager, and perhaps not the one who normally feeds and cares for the ducks? Not terribly significant, except that I've noticed birds, and animals in general, can spot a youngster and their unpredictable behavior and wisely be on guard.)
Meanwhile, and regardless of the "why", you need a solution, and I'd like to offer something to try that has NEVER failed:
We have peafowl, guineas, geese, ducks, and chickens. (We've also kept turkeys.) At one time or another, the various males will get their feathers ruffled and become aggressive. Mating season is a great occasion to witness this behavior. Feeding time, between breeds, is another.
What works without fail, regardless of breed is "stalking" the aggressive bird at a quick pace. This takes a little bit of nerve, if one, such as your son, might be timid. If that is the case, do it with him. If he is not timid, do this with him at least the first time or two.
Say you are going out into the yard (plan this so it doesn't interrupt something else you planned, thus resulting in this not being effective). When you go into the yard, spot the drake, and begin walking after him at a quick pace. Do not harm him, or kick him, or any such thing, just walk straight at him. Follow him around the yard for a few minutes doing this. Do this a few times the first day.
Repeat for a few days, at least once in the day. I PROMISE this will end this behavior. Anytime it looks like the drake has an eye for challenge, stalk him before he has an opportunity.
This works for all the birds. If your son is SMALLER than the birds, YOU need to do this.
This is something we observed in nature, namely from the favorite peacock who would stalk an aggressive rooster that challenged other roosters. The peacock's stalking in essence disallowed any other bird from being aggressive or fighting. When there is no peacock around, the flock needs me to keep them in line. I rarely have to do it, but if there is an aggressive bird, I stalk. Three days tops.
Please let me know how this works for you!