The head to the ground neck extended running is threatening to run the recipient of the behavior off the property. I have two meanie pekin drakes that chase away one of my muscovy drakes using that action.
The whole behavior you describe does sound, as
azygous said, like hormonal aggression. I have a muscovy drake [not the one chased off by the pekins] that gets very aggressive with me -- he attacks my feet and lower legs when he is in a bad mood. I put a dog crate in the garden near my house and when the muscovy comes to attack me, I pick him up and put him in the dog crate for 2 or 3 minutes [longer unnecesary]. He comes out chastened and well behaved until he is next in a mood and attacking my feet when he is back into the dogcrate!
Can you use this sort of discipline with your boy?