Interesting about the teeth (and gross). I've always used the lower sugar version of gatorade and added salt (doctor's recommendation). I'm not sure that vitamin water actually has less sugar anyway, does it -- unless of course you're buying the low-sugar vitamin water?
Okay, this isn't so much a solution as a possible bridge to something better or even just a "lesser of 2 evils" thing, but how about diluting the gatorade half and half with water. I do that sometimes with vitamin waters or other artificial tasting drinks when there aren't alternatives, or sometimes even with sugary juices. I normally do about a third of the cup juice and then put in ice cold water, but you could always start with half and half. He won't like it as much as regular gatorade, I'm sure, but he probably won't DISLIKE it either, and it's a way for him to make a compromise that is at least a step in the right direction toward a less sugary diet.
Otherwise, maybe pick you battles first in other areas. Does he have another sugary sweet or food that he he eats often and could wean down on. My nutritionist always says to cut empty sugars first, obviously.
