Is the money given by the father to his son who obtained a master's degree lawful for him, given that the rest of his siblings did not receive the like because they did not earn the same qualification?

A father must be equitable among his children in giving gifts, unless there is a valid reason for تخصيص (singling out), such as need, chronic illness, blindness, a large family, or engagement in seeking knowledge. And your father's gift to you is permissible as an appreciation for your pursuit of knowledge.