Mean Roeon

The "pecking" order has been turned upside down with new ducks added to the mix.  how old are your new ones?  if they are youngins I'd separate them from him. Are they all girls and just the one drake?

If the ducks are female that he is bothering, it sounds like he is mating because that is what they do.  One of my drakes continually gives one of the others a hard time and it looks like he is trying to mate as well.  Unfortunately drakes arent very kind when mating.

Re underweight, they may need worming, its a start anyway.  I have just bought some worming and red mite medicine (as my chickens had mites), they had been wormed but did them again for all types of chickens, ducks etc and you put the stuff on the back of their necks.  I will get the name of it when I go down the shed next, I bought it online and I live in Australia but Im sure you will be able to get something similar.  leave it with me and I will get the name but in the meantime check online.  They love lettuce, bread and fresh spinach leave which is very good for them so that should also help.

Epernix pour-on is the best over here as far as killing internal and external parasites, and no egg withdrawl.

It really does sound like he's breeding them. That is not mean, it's entirely normal behavior. If he's the only drake, he will likely settle down in a few days. You may want to limit his access to short periods until he's calmer though. At night he does need shelter, food and water, can you rig him up a better confinement area?

As far as the underweight ducks, it is best to have a quarantine area for new poultry to keep them in for a couple of weeks before you bring them home. They may have a disease or parasites that will affect your other poultry. They need plenty of good food and shelter - be aware of their feather condition. If their feathers are poor and lacking oil, they can get soaked through and that would be really hard on them. I would worm them. You should get veterinarian advice, if your vet knows anything about poultry. People on the internet often want to be helpful, but may not really know what they are talking about. Be sure to do your research - ultimately you are responsible for your pets.

Obviously you don't have a quarantine area set up, just thought I'd mention that so that next time you can be more prepared. If you have a coop area, maybe you can partition some of it off for the lonely drake? Even a dog kennel can be ok, but do get him some food and water and let him out in the morning. You can put food in any plastic bowl or clean left over food container. Usually you can find some way to give a bowl of water too.

Good luck with your ducks!