When male ducks don't have an outlet for their hormones and aren't able to mate with females, they can and will start trying to mate with each other. They will chase each other down, attempt to pin each other to the ground, hold each other by the backs of the neck (which you are seeing result in feather loss and wounds) and then try to forcibly mate with the male they've caught and pinned down. Back when I had just a pair of males as pets, they would do this to each other, but only occasionally and it never resulted in injury. Yours however are injuring each other, and thanks to selective breeding, their mating 'season' now lasts nearly the entire year, so they will keep up this behavior for almost the whole year until their hormones subside temporarily in the winter.
As you've stated, you can't keep the one male alone for the rest of his life, that's not fair. You can't put him back in with the other males or he will continue to injure them. You can't get more females. You can't put him in the group with the females because you already don't have enough females for the one male (best ratio is 3 to 4 females per male) so adding another male could easily lead to the females becoming overmated, stressed, and injured.
You're left with only one option and although it stinks, it sounds like you need to find him a new home, preferably with some females he can call his own. I know it's not fun, but you have to put the needs of the animal ahead of yours and do what's best for him and not what's best for you. That's what responsible animal ownership is.
