I am new to this board, but not to iVillage.
Is it that he won't (or possibly can't) drink MILK, or that he doesn't drink ANYTHING?
Does he use his own sippy cups/cups at daycare. That might be helpful, also I would fill them with water/milk from home. I know sometimes one of my kids will refuse water and/or milk b/c it doesn't taste like home. Another idea is that if he has complete access to water at home, it might be limited to meals at day care.
Hope these ideas help, maybe even enlisting his help in selecting a new sippy cup for daycare.
I would say, being that he's two... and he drinks fine at home... that it wouldn't be a concern. Just make sure he drinks something before he goes there in the morning... and then resume as usual when you get home. I can't imagine them dismissing him for not drinking anything... if they do - you probably don't want him there anyways. I guess I would just encourage them to keep offering him drinks... but not worry if he doesn't take them. If he's thirsty enough, he'll drink something ;)