I think the moral is about the boy, not the villagers and what they believe - it's that reputation is an important thing, because once you damage yours, you won't be able to use it even when you really need it.

If I can jump in here: it's something like "be very careful when making extreme claims, because they probably won't be believed. If your claims turn out to be false, then they really won't be believed next time."

So it's about trust and reputation. If the village elder, whom everyone trusts, cried "wolf!" the villagers would be inclined to believe him.

Don't have a small child be responsible for protecting your sheep from wolves? Esepcially if you have already determined not to come if he shouts for help.

Don't lie so people can trust you.

If you keep lying or making false alarms, people won't believe you when you're actually in trouble. So, it's better to tell the truth and be trustworthy, or you'll end up losing the trust of others when you really need it.

TL;DR: Don't lie, or trust will be lost.

That you should never tell the same lie twice.