If your choices are the following: After hearing a series of lectures on supply-side economics, my father had great respect for him. Jordan joined the band, which was a surprise. Grace gave my little sister a book of illustrated children's stories that was her favorite present. James wants to study medicine because his brother is a doctor.
Then the answer is James wants to study medicine because his brother is a doctor. In this sentence it is the most clear that the pronoun his refers to the subject James.
