You'll need more females than just two! He will eventually kill therm with his attentions, my lad has four girlfriends .

If they have a varied diet they won't need vitamins - mine get layers pellets and are free range on a grass and shrub paddock with a wee burn running through it.
Do you have a bath for them (or a pond or stream) - they need to dip their heads in water to oil themselves as well as for drinking.
Mine don't get water at night - they make a big enough mess in their shed with theri watery poop as it is. I have plastic sheeting on the floor so I can just pull it out, hose it down, leave to dry and replace.
I've never noticed my ducks getting any beasties. I think you'd know - they'd scratch a lot.
Have fun - ducks are fantastic, and so are the eggs!