Is it always the same one that starts the bullying? It may be the others are just following suit and then ganging up? I know I read with chickens you should remove the bully rather than the bullied and then slowly re-introduce. It may be if he gets some bonding time with the rest, one of the others will start standing up for him and then less likely to get as brutal in future.
This is all theory as my 4 female ducks haven't fought, but this is what I would be tempted to try, otherwise it would mean creating two group as it isn't fair if he is really getting ganged up on.