I'd suggest that you pour a juice into a cup while he watches and then add water. You can do a 50-50 combination. And you can tell him he has two choices. He can:

1. Drink the combination
2. Drink water only and then juice only

Give kids choices they can't refuse AND that satisfy your requirements and voila! Everyone is a winner!

I had this problem to.. I discovered this new brand of water called wat-aah that the kids love and I put a lemon, lime or orange wedge in it and it always gets drank.

Ok he likes the watered down juice but i still want him to drink some sugar free stuff... I might try that wataah stuff.. I found the site for it.. pretty cute.. www.drinkwataah.com/