Your drake has got stuck in aggressive destructive ways. He has to go in drake jail. Can you separate him where he cannot see or hear the other ducks? Is there a female he hasnt attacked that might move in with him under supervision?
I rescued 2 juvenile pekin drakes in April. Both were in a very sorry state (dirty, thin and dehydrated) but one had lost the feathers round his left eye and down his neck. The two were very tightly bonded and stood with their necks wrapped round each other when ill at ease, but it quickly became aparent that one drake was abusing the other. The behaviour didn't stop despite separation during quarantine and rehab. Therefore I separated the two. The aggressor went to my son's flock for about 10 weeks. When he came back, he was still tightly bonded with the second juvenile drake but no longer able to bully him. The feather pulling and biting did not restart. So I think, if your drake is separated for a couple of months, it might break his aggressive behavior