J.K.
What about chocolate milk? Juice with or without sugar is bad for them. They are only supposed to get 4 oz a day max of 100% fruit juice. A watchdog group recently found high levels of lead in juice. I'll post link to an article on it at the end of this post; if you google "juice lead", you'll find tons of articles on the topic.
Crystal Light is sweetened with aspertame which has a whole list of possible side effect. Personally, it gives me anxiety. I don't know a lot about Purevia, but the FDA keeps banning and unbanning it - I'll provide another link below.
Try flavoring his water with a fresh squeezed orange or lemon or put a slice of cucumber in it.
Sounds like your son needs to be on a high fat diet. My 26 month old weighs 26 pounds and he is in the bottom half of the growth curve for his age. His nutritionist wants him on a high fat diet. Lots of whole milk, cheese, avocado, high fat yogurt (Trader Joes sells some), coconut milk, etc.... Or, even though it has a lot of sugar, Pediasure could help put some weight (and liquid) on/in him.
Oh, and btw, sugar substitutes are not bad for teeth. Only sugar eats away at teeth. But, they are bad in many other ways.
http://www.huffingtonpost.com/maria-rodale/is-there-lead-...
http://nutritionwonderland.com/2009/02/stevia-controversy/