how could Jesus be called the Son of Man before His Incarnation?
Traditional teaching is that The Word of God is not The Father, but was with The Father, and was Himself God. Thus, there are created beings (eg, "watchers of men"), who have existed alongside Christ before His human incarnation as Jesus. There are spirits who speak alongside The Word of God, yet do not represent God. Discernment is required of a Christian to recognise The Holy Spirit. The Word became flesh and dwelt among us, that we beheld His glory.
Ephesians 4:9 shows that St. Paul understood The Son as having descended to earth.
The difference being that Jesus did not have a human body of His own before the incarnation (according to our knowledge), so The Word of God would speak spiritually where the physical manifestations of creation (prophets, donkeys, burning bushes etc) would assist His message to strike our heart.
By becoming human and speaking words of His own volition, He has achieved John 15:22. Yet, the world did not receive Him, rather in their heart was a weakness of sin leading to covetousness (Matthew 21:38-39). So the world was thrust into darkness (John 12:35). Because of this, that He was cut short before having achieved completeness of His Messianic duty (Daniel 9:26), He has established the Alpha of The Kingdom of God, that we carry forth in His name (Luke 12:49-50) while He reigns from heaven as before, but having personally established His Word in earth.
For all things have been created through Him, and for Him.
Psalms 8:4 is one example of the phrase "son of man" - showing that the prophets used the phrase "son of man" to describe a servant of God, a human who is reverent and pursuing faith in Him. So, Jesus called Himself the "son of Man" in the same way, to take on that identity and to take that identity to the cross such that no other son of man could do (Ecclesiastes 3:18), so that we sons of men who follow Him, can be empowered to carry a new title in His name through having been bonded to Him through the covenant of our baptism. When Jesus' work is perfected in us (Hebrews 10:14), we are empowered to be called "sons of God". (John 1:12-13, 1 Corinthians 6:3, Revelation 19:9, Romans 8:15, Revelation 21:2).