Firstly, I think you meant "dyeing" poison dart frogs, not "dying" poison dart frogs.

Also, it can be really difficult to tell what is wrong with poison dart frogs unless you post pictures.
It could be any number of things at this point, and there's no use jumping to conclusions without pics.
Does he have any other symptoms? What's the temperature? How high is the humidity? How old are the frogs?
Yes, skinny waist and hind legs can be a sign of a serious problem, but not always.
Again, any pictures would help greatly.