The moral could be to think before you act. In the case of the boy trying to catch the fish, if he had thought about how fish move and what he needed to catch them, he wouldn't have made such a silly attempt.
The moral can be about being prepared. Just like the boy on the hike. If he had been prepared with water and snacks, he would have had a much better time. It shows that not being prepared can lead to a lot of discomfort and problems.
The moral could be about perseverance. The blind boy might face many difficulties but still manages to carry on with his life. This shows that no matter what hardships we face, we should not give up easily.
Since I don't know the story specifically, it's hard to say for sure. But surely the stupid boy himself is a main character. There might also be his family members, like his parents or siblings who could be involved in the story, either trying to help him or getting frustrated with him.
One possible moral could be about the beauty of different worlds coming together. The boy from the human world and the mermaid from the underwater world meet, and it might teach us to be open to new experiences and different kinds of beings.
The moral could be about kindness. Maybe the boy shows kindness to the rabbit and in return gets a positive outcome, like friendship or learning an important life lesson.
The moral might be to think before you act. Since the boy is foolish and does things without considering the consequences, it could teach us to be more cautious.
The moral is that if you lie too often, people won't believe you even when you're telling the truth. In the story, the boy keeps crying 'wolf' when there is no wolf just for fun. So when the wolf actually comes, no one believes him and he suffers the consequence.
The moral could be about kindness. If the boy was kind to the frog in the story, it might teach that being good to others, even different ones like a frog, is important.
The moral is often 'Beware of the evil in disguise'. Just like the snake in the story, it might seem harmless at first but can turn out to be very dangerous.
The moral could be that talent can be found in unexpected places. Just like the boy with the violin might have a hidden gift for music that others initially overlook.
The moral could be about selfless giving. The tree gives everything it has to the boy throughout his life without asking for much in return. It shows that true love and friendship involve sacrifice and putting others' needs before your own.