They aren't that lethargic, try the sugar water which is a quick form of carbohydrates. Being a type 1 diabetic I'm all familiar with how low blood sugars can feel, and brain damage can set in if it's not fixed. If it doesn't improve there could be something wrong with it's development while in...
The only thing that might help is to get some sugar water or electrolyte solution into it, I use a teaspoon of brown sugar to a quart of water, but I might mix it a bit heavier and see if I could get some drops down it, if it's going to help it should within an hour or so, it may have low blood...