E.R.
Hello.
My daughter's pediatrician recommended vitamin supplements only in the instance that she became a picky eater. Sounds like your little one fits the bill, so you should probably get an infant supplement for your son.
As for the water situation, my daughter really dislike water at five months old, even though we had to give her some at the time due to the fact she had a stomach flu. I would suggest to keep trying. Eventually, your child will drink water after he's acquired the taste for it. If it means you have to spoon feed your son water on occasion, then that may be the only way for him to drink water for awhile. It takes time.
As foods and solids are concerned, sometimes it takes up to ten times before a baby will like a particular food whether it be vegetable, meat, cheese, or whatever you choose to give. I would just try to expand the things your feeding him, starting with veggies first. Once you start on fruits, sometimes, it's hard to get them on the veggies because they develop a taste for sweet things.
Once your son has a much more varied diet, you won't need to give him supplements.
All you can do is try and keep on trying.
Hope that helps!
E.