You should not accept the word Son literally, because many Prophets and we people are called in the Bible also, sons and children of God. Read Exodus 4:22. "And thou [Moses] shalt say unto Pharaoh, Thus saith the Lord, Israel is my son, even my firstborn."
Here is Jacob (Israel) His firstborn son.
Read now II Samuel 7:13-14 or I Chronicles 22:10. "He [Solomon] shall build a house for my name, and I will establish the throne of his kingdom for ever. I will be his father and he shall be my son..."
It will be confusing if you read Jeremiah 31:9: "1 am a father to Israel and Ephraim is my firstborn." In Exodus 4:22 just now, Israel was called firstborn, too. Who is now the real firstborn? Israel or Ephraim?
Common people can be children of God; read Deuteronomy 14:1.
. "Ye are the children of the Lord your God." Common people can also be called firstborn; read If all are firstborn, what is Jesus then?