The relationship between father and himself should be filled in with the relationship between father and son. The popular understanding is the corresponding position relationship between him and you in the family. He is the father and you are the son, so fill in the father-son relationship.

Filling in the relationship is different from filling in the title. The relationship between a son and his father should be written as father and son or parent and child. If it is an index question, then write father or father because it refers to father alone, so fill in father and son.