Kaito did not know if his uncle was being truthful when he promised to let him leave. However, he had come to understand the man. While he could not like someone who kidnapped him, he knew that the man did not have evil plots against him.
Taiki's only weakness was his desire to help his family. Unfortunately, Kaito was the collateral damage to keeping his family safe and secure. So, Kaito decided to agree with him instead of leaving and being followed by another powerful Guardian.
If he were only concerned for himself, he would take the chance of attempting an escape from the island, even at the risk of being a target. However, he could not leave Akira vulnerable to the people of Martial Island.
