you have to be stubborn, right at the the end in the ice cream shop, there's a dialogue option where you answer one way and Brandon will just keep saying no and take you back to the dialogue option. But if you keep answering that way eventually Brandon breaks, tells you that the girl may not want to breed with you after telling his story, and then you make him tell his tear jerker story. You may need to progress the Natasha questline first where he's answering questions naked on a couch but I'm not sure about that