Find bottles with cartoons that he likes or finds interesting. Bring him out and let him pick a bottle on his own then explain to him that now that he has a bottle, we will drink water together. Leave opened bottles or cups where it’s accessible for him to see and reach. You can leave a few so that he will remember he needs to drink water. That’s what I did for my lo, I will put bottles and cups at places where she usually go to, then she’ll pick it up halfway and drink then back to play. Can also offer him water when he’s distracted with toys/books. Try adding 1-2 berries (cranberries, blueberries, strawberries etc etc) inside the water to attract him.