If the reality is exactly as what is mentioned in the question, the underaged boy should have a Shar`y (Islamic legal) Waliy (guardian) as defined in the will of his deceased father. However, if his father did not specify a Waliy; the Shar`y ruler should appoint one for him.
This Waliy has the responsibility before the Shar`y ruler to manage the property of the underaged boy. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.