I've got a mean mallard drake, too. And nope, he never does learn to get along with those he picks on. With our two girls, he will mate with one (as gentle as a drake ever mates), with the other, he will mount her, but it is only for dominance. He'll tear at her neck and back, and never actually really mate. If left together for enough time, he will strip her neck bare of feathers, and bloody her up.
He can go through periods of not being too aggressive, so we feel comfortable leaving them together if they're wandering the yard. Or in the cage together, if it's only going to be half a day, but not more. But no way, no how, can we leave him with the one he hates, for more than a day at a time. And he has never grown out of it.
Outside of the mating season, say November through January, we can extend the time he's together with her. But we usually won't leave them together. (We put the two girls together on one side of the cage, and him behind a door on the other side, and we have a screen in the middle of their shared coop.)
With the other girl, however, no problem. We can leave them together all the time, and he never gets too aggressive. It's the usual duck thing that it looks rougher than you'd expect, but he won't pick her neck bare and bloody like he will with the other.
So, I vote with watch him. It could be something he never gets over.