In Genesis 45:8 אב would be better translated in English as "chief counselor", "confidant" or "authority", rather than the somewhat misleading "father". Joseph is expressing the fact that he has both Pharaoh's trust and ear.
In this verse, Joseph speaks to his brothers in their own Hebrew idiom. He is not speaking to them as an Egyptian. His brothers are, after all, simple Hebrew herdsmen from Canaan who do not understand the Egyptian language or the Egyptian administrative structure, so there is no reason why Joseph would use a technical Egyptian term of rank that his brothers could hardly be expected to understand.
The word אב, "father" is the first of three expressions of authority in this verse, the other two being "lord of all his household" and "ruler of all the land of Egypt". If "father" were being used as a term of rank in this verse, then we would expect that "lord of all his household" and "ruler of all the land of Egypt" would also have to be terms of rank, but they are obviously not. The style of this verse, with it's triple emphatic parallelism is colloquial, not official or technical. Joseph is not boasting about the titles that he holds, which titles would likely mean nothing to his brothers, he is revealing the miracle that God has placed him in a position of authority.
Furthermore, in Genesis 41:39-45 where Pharaoh himself speaks and appoints Joseph, Pharaoh does not use either the word "father" or any other word that could vaguely be construed as an Egyptian term of rank. In Genesis 42:6, the text refers to Joseph as "the ruler" and "the provider", but not "the father".
The usage of אב, "father" and אם, "mother"" in Hebrew and other Semitic languages is much wider than "father" or "mother" in English, and is often used figuratively. The figurative usage in Genesis 45:8 is plain from other verses in the OT itself listed below.
Judges 17:10 (NIV):
Then Micah said to him, "Live with me and be my father and priest, and I'll give you ten shekels of silver a year, your clothes and your food."
Judges 18:19 (NIV):
They answered him, "Be quiet! Don't say a word. Come with us, and be our father and priest. Isn't it better that you serve a tribe and clan in Israel as priest rather than just one man's household?"
II Samuel 7:14 (NIV):
I will be his father, and he will be my son. When he does wrong, I will punish him with a rod wielded by men, with floggings inflicted by human hands.
Isaiah 22:21 ():
I will clothe him with your robe and fasten your sash around him and hand your authority over to him. He will be a father to those who live in Jerusalem and to the people of Judah.
In Genesis 17:4 (NIV):
As for me, this is my covenant with you: You will be the father of many nations.
there is a double entendre which unfortunately does not translate into English. The word אב is used in both the figurative and the biological senses. The promise is that Abraham will both have a multitude of biological children and will also be the guide of many nations.