I am confused about Aristotle's virtue ethics as it applies to Aesop's fable of
Perhaps you're putting more weight on the fable than fables are meant to bear. Fables are short, stylized ways of conveying a point, and the point here seems clear enough: if you come to be seen as a liar, you risk not being believed when it matters.
Though I gather that there's a remark attributed to Aristotle that conveys much the same message, once again it may not be profitable to put too much weight on the fable as a device for exploring Aristotle's Ethics. The boy seems clearly to represent someone who lacks an important virtue: truthfulness. The fable doesn't really address the question of whether the townspeople also lack some virtue such as prudence or caution. In real life, we might wonder how many times the boy would have to lie before it become reasonable for the townspeople to ignore him; in the context of the fable, that's not really the point at issue.