Son of Man was the favourite title of Jesus. It has multiple meanings. It can mean, as it often is in Ezekiel, human being. But Daniel 7:13 describes a vision of divine being who is one like a son of man. Jesus seemed to have used it with the double meaning. It was not as controversial as “Son of God” but it did point toward his divine identity.
The fact that Son of Man is very prominent in the Gospels but was not popular in the early church, suggests that this goes back to the historical Jesus and was not an invention of the church.