hmmmmm i would just keep offering him his cup just to make sure his fluids are ok - you can also feel if he is dehydrated as his fontanelle will feel sunken, but its unlikely. the ratio of bf/food goes up and down in my experience and eventually he will just prefer food
i dont think there is a set amount of water/fluids he should drink and things like yogurt have water in them too, i would just keep offering.
i honestly can't remember when we stopped boiling water....it was before we moved so he cant have been older than 8 months. i think once they are moving around a bit and putting literally everything in their mouths there doesn't seem any point in boiling it - i vaguely remember that we only really boiled it carefully when elijah was under 6 months.