Since he is on breast milk, it has everything including water in it in a more balanced way for the baby. If he is not constipated then he should be fine without additional water. You could try adding a small spoonful of lemon juice in it just for the taste sake. Try adding a piece of dates, raisins, pinch of sugar or something just to enhance the taste.
Again consult your pediatrician and if he still says there is no need for all this then go with him/her. They know best!

Adara