Maybe because the boy is young. People tend to think that young children might not be able to tell the truth accurately or might be exaggerating things. Also, if daddy has a good reputation in the community, people are more likely to side with him rather than the boy at first.
Well, 'R茅mi Nobody's Boy' is not based on real events. It's the product of the author's imagination to engage and captivate readers with a fictional narrative.
It could be that the story was just too far - fetched. For example, if he claimed to have seen a unicorn in the middle of the city. Things that seem highly unlikely are hard to be believed.
Another possibility is that Remi's Secret Story has to do with a secret identity. Maybe Remi has a double life, and she is involved in some kind of activity that she doesn't want others to know about. For instance, she could be a volunteer for a secret organization that helps people in need, and she has to keep her involvement a secret for safety reasons.
The boy could bring in a reliable witness. If someone trustworthy can confirm his story, others are more likely to believe it. He needs to find someone with a good reputation who was also there or has knowledge of the situation.
I'm not sure specifically which 'Remi' and 'Secret Story' you are referring to. There could be many Remis with their own stories. It could be a personal story about a person named Remi, or it might be related to a fictional character in a book, movie or game.