Since he's such an important person, trying to convince him with simple things doesn't make sense. So I would like to let him know that we already have several small towns willing to participate, but that the larger towns are waiting for him. I would convince him by demonstrating that once you have these defenses could begin to make a counterattack getting many resources and that a large part would go to his people and also making him excited to know that it would be one of the most important in this defense becoming almost a hero in what represents humanity against the horde.