A.V.
My DD could live on grape juice, but she fills up on drink and won't eat. What we do is give her a half glass to start the meal (I had a babysitter that wouldn't give you a drink til you ate the snack, no matter how dry, and I hated it) and if she finishes that, she has to eat some of her food before she gets more to drink. And sometimes I say, "You've had plenty of juice today. Do you want milk or water?" With my DD, milk is often flavored with a little strawberry Quik and water gets ice cubes, which she thinks is fun. If I give her another choice that is "fun" she fusses less about the juice. So I'd keep him hydrated (it is HOT HOT HOT here) but try to balance it with snacks or meals so it's not just milk.