Son of man

Jesus... he was a son of Josef and Mary so naturally he was a son of man. And he described himself as a son of man many times, he didn't say he is a son of God even one time. If I remember correctly he was asked by Pilate whether he is a son of God, he said just 'as you said so' . This is a form of negation.
Christianity was started by people who believed Jesus is a son of God.
At that time the title son of God was used to refer to Octavianus, Emperor of Rome.
Actually a son of God is as good as the alternate name for king.

But if you think the title son of man is a humble expression, you are misled.

Is any ordinary person claim on purpose he is a son of man?

What Jesus wanted to connote was that he was originally a son of God but he was born from human parents.

Thus a son of man is more glorious than a son of God in meaning.