Why did the NWT change it to "friend" if "son" always means "son" like you are saying?
First off, I never said it always means it. Also, it is not just the NWT that changes it to 'friend' but also others say, 'anyone', 'man', 'person', 'someone', etc.
But just as you contradicted yourself as shown here in your own words:
one cannot argue that the expression "son of X" in the Bible means "child of X,"
Yes, there are times the word "son" refers to a male child,
When it is speaking of Jesus being the 'son of God', are those times.
As well as:
Galatians 4:4-6 [But when the full limit of the time arrived, God sent his Son, who was born of a woman and who was under law, that he might release by purchase those under law, so that we might receive the adoption as sons.]
John 20:17 Jesus said to her: “Stop clinging to me, for I have not yet ascended to the Father. But go to my brothers and say to them, ‘I am ascending to my Father and your Father and to my God and your God.’”
And here.