I think I've seen this in cafes, if I'm right it has fruit juice in it as well as water, so it has natural sugar, but I think it's less sugary than pure fruit juice. Check for how much sugar content on the label - normal orange juice has 10g per 100ml. If you don't carb count then I would suggest he could have a small amount with a meal, but it's not something I would drink on it's own without covering it with extra insulin.