You can get the vitamin supplement without fluoride.
A child only needs fluoride from 1 source, or it will be too much duplicating/over supplementing. And too much fluoride is not good.
You get bottled water.... thus purified, right? Good.
Yes, some tap water in some areas are bad. I would not want to drink that stuff myself.
In each case, tap or bottled water... I would ask how much fluoride is in it. But just omit the tap water route....yuck!
In my state, the water is not fluoridated...thus I give my kids supplemental prescription fluoride drops. That is what I had as a child, and all my life I've only had 1 cavity gotten in adult-hood.
Ask your Pediatrician, and TELL him/her what your child's Dentist said.
I would give him bottled water, and a vitamin supplement, sans the fluoride, if you are getting the bottled water WITH fluoride. And then too, toddler toothpastes have fluoride in it too (it's a topical versus an ingested fluoride).... and some kids drink a lot of water... so just HOW does one determine how much fluoride a child is getting?
Going more conservatively is probably better... better than over-doing the fluoride. But NO tap water.... in your area, it sounds pretty bad and yucky.
Good luck, so confusing, eh?
Susan