Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 113.
In the New Testament Gospels, Jesus used the term “son of man” in three different ways. On some occasions he uses it simply as a circumlocution for himself, that is, rather than referring directly to himself, Jesus sometimes speaks obliquely of “the son of man” (e.g., Matt 8:20). In a related way, he sometimes uses it to speak of his impending suffering (Mark 8:31). Finally, he occasionally uses the term with reference to a cosmic figure who is coming to bring the judgment of God at the end of time (Mark 8:38), a judgment Mark’s Gospel expects to be imminent (9:1; 13:30). For Mark himself, of course, the passages that speak of the coming Son of Man refer to Jesus, the one who is returning soon as the judge of the earth….[S]cholars debate which, if any, of these three uses of the term can be ascribed to the historical Jesus.