Caren, I don't know much about anything, really, but I would think anything "qualifying" as a liquid could count as "fluids".
Be imaginative. I would include ice cream and soups, for example, in the fluid count. I don't see a problem with diluted fruit juices. Whatever he will drink is good.
Even beer or wine can be an acceptable choice, unless he drinks enough to cause a medication reaction. It's a liquid and even comes with the added bonus of calories!! LOL
I guess my point is, whatever he will drink is what you should go with. And some drinks can be "boosted" with a bit of nutrition, too. (Like protein powder in a shake.)