This is some awesome thinking!
He didn't know what to say to them since they refused to believe the truth. He explained again and again, and they dismissed his explanation as a figment of his imagination. There was no way that grandpa, who had been dead for five years, could have told him where the treasure had been hidden. Of course, it didn't help that grandpa was roaring with laughter in the chair next to him as he tried to explain once again how he'd found it.