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!
