The boy told him to help end the war but he responded it wasn't so easy, because, though he was a Crown Prince, he had next to no say in the court. The boy suggested he make a big achievement in order to boost his name and force the court officials to listen to him. When Jing Lan asked him how, that was when he revealed his identity. And there, they made a pact, the boy would help Jing Lan make a name for himself and Jing Lan would defeat his mother's enemies so it'll be smoother for him to seize the throne.