How does it not make sense? Are you somehow assuming that kids are keeping an ear out and listening to their mothers 24/7? I'm sorry, but that makes no sense. And yeah, the way he reacts is believable. She says she saw a dragon. Not everyone has. Everyone knows that dragons haven't been seen in centuries, and they're just now coming back. It's also the very start of the game. Nobody's going to believe you about dragons at first. Some people will deny it when everyone says there's dragons about because it's crazy talk and denial is a way to deal with things that you don't want to be true.
Mom: I saw a dragon!
Kid: What? What is it now, Mother?
Mom: How are you?
Kid: What? What is it now, Mother?
Mom: Hello!
Kid: What? What is it now, Mother?
Mom: Goodbye.
Kid: What? What is it now, Mother?
Mom: Have you heard that X was attacked by Y?
Kid: What? What is it now, Mother?
Mom: I hear that blah blah blah is very useful with a blade.
Kid: What? What is it now, Mother?
Mom: I saw a dragon!
Kid: What? What is it now, Mother?
Mom: Goodbye.
Kid: What? What is it now, Mother?