A 12 month lease expires on the 365th day, not sooner. He can only move in if he and his family are going to live there, not a new tenant.
Are you sure you have a lease? What did you sign? The dispute is between the owner and the agent, not you at all. You must not deal with the owner at all. If the agent made a mistake, it is not your problem. Tell him to go away and speak to the agent.