Jesus was evidently not a human being before being placed in the womb of his human mother.
It is not said that Jesus is "son of man" because he maintains his human nature forever, but because at some point of his life he was also human and in that same human nature, he lived for about 33 years being obedient to God, and as a human he was thirsty, hungry, etc., and more than that, he allowed himself to be murdered by God's enemies without failing God.
Heb. 2:9 But we do see Jesus, who was made a little lower than angels, now crowned with glory and honor for having suffered death, so that by God’s undeserved kindness he might taste death for everyone.
Now Jesus is no longer a human, because the physical human nature that he had was inferior to the nature of the spirits. If he were still a human, God's enemies would be more physically powerful than him and could, just as when he was on earth, cause him physical harm. Likewise, if he were a human in heaven, he could not live without satisfying human physical needs such as breathing, drinking, eating, etc.
Heb. 1:3 (...) after he had made a purification for our sins, he sat down at the right hand of the Majesty on high. 4 So he has become better than the angels to the extent that he has inherited a name more excellent than theirs.
Jesus is now not a being of flesh and blood. However, the Bible sometimes refers to him as "son of man" to remind us that he was obedient to God when he was equal to us, and in that state he learned to obey God under natural physical trials and necessities, which In that state he paid the price for his human brothers, offering his life when he shed his human blood until death.
Now Jesus is an even more powerful spirit than he was before he was human, because he was rewarded for obeying God until death in a weaker state, that of humans.
Heb. 5:7 During his life on earth, Christ offered up supplications and also petitions, with strong outcries and tears, to the One who was able to save him out of death, and he was favorably heard for his godly fear. 8 Although he was a son, he learned obedience from the things he suffered. 9 And after he had been made perfect, he became responsible for everlasting salvation to all those obeying him, 10 because he has been designated by God a high priest in the manner of Melchizedek.
So in Jesus' history or record of fidelity, his loyalty in a physical nature similar to ours is added. So when it is said that Jesus is "the son of man" it doesn't mean that he is still a human being, but it refers to the fact that he received the reward after having been one of us: the second Adam.
Phil. 2:5 Keep this mental attitude in you that was also in Christ Jesus, 6 who, although he was existing in God’s form, did not even consider the idea of trying to be equal to God. 7 No, but he emptied himself and took a slave’s form and became human. 8 More than that, when he came as a man, he humbled himself and became obedient to the point of death, yes, death on a torture stake. 9 For this very reason, God exalted him to a superior position and kindly gave him the name that is above every other name, 10 so that in the name of Jesus every knee should bend—of those in heaven and those on earth and those under the ground— 11 and every tongue should openly acknowledge that Jesus Christ is Lord to the glory of God the Father.