No, not necessarily... Kittens often refuse to drink water when they are not used to it.

I thought he may already be dyhydrated because he seems to be weak as he's sleeping next to his litter box now. Is his nose dry and warm? And when you take his skin between your thumb and forefinger, is it elastic, or does it take a couple of seconds before it "unwrinkles"? Are his gums pale?

However, this sounds like a serious condition to me and you shouldn't hesitate to bring him to a vet!

Kirsten