This is a really deep question, isn’t it! Many persons have puzzled over this for many years.
The Bible tells us that Jesus isn’t ‘God’s human son’, rather, that Jesus, who is co-eternal with God became human. Jesus. like God the Father, is eternal and perfect - but he became human so that he could enter into our world and die for humans. The Bible treats this as something to marvel at:
"Jesus Christ ‘though he was in the form of God, did not count equality with God a thing to be grasped, but made himself nothing, taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross." (Philippians 2:6-8/ESV)
The mystery of, "How can Jesus be both God and God's Son" is not completely explained to us, but we are left in no doubt that Mary’s conception and pregnancy is a miracle from God. (See Luke 1). And then, in the gospel of John, which is written in very symbolic language at the beginning, speaks about ‘the word becoming flesh’. ‘The word’ is Jesus - who was with God in the beginning - and he became flesh - this is what ‘incarnate’ means - to be made flesh. This makes it clear that Jesus is the eternal God becoming human. So, it isn’t so much that God had a human son, but that God’s son became human for our sake.
May God marvel us forever!