There's no real need for water at your LO's age, just let him have milk whenever he wants it, BF or FF.
I just left water out for LO to have a go at, in a variety of different cups. Eventually they'll realise what it does. Having cups to play with in the bath helped too - I swear my kids got half of their daily fluid intake from bath water and it was great at teaching them to use a normal cup.