Does it mean that he does not belong to the family (or to your group of closest people, friends, relatives, neighbours, etc.) or that he is someone you don't know or both? (Maybe it is because I'm thinking in Spanish, where familiar means both family and familiar, and that οἴκος meant both house and family, but family in the sense of the Latin familia, which included both relatives and servants).
Does αυτός here refer potentially to something, not necessarily a person? "It is not familiar," with "it" referring to the grammatically masculine antecedent? If it refers to a person, is it better to use γνωστός or are the two more or less used interchangeably? Οικείος in ancient Gk referred to kin, family, so it makes sense to me if it can refer to a person in modern Gk, but maybe it's better to use γνωστός?