who is the son of man?

Son of Man is an ordinary phrase for "human being." 

The Hebrew expression "son of man" (בן–אדם, ben-'adam) appears 107 times in the Hebrew Bible, the majority (94 times) in the book of Ezekiel. It is used in three main ways: as a form of address (Ezekiel); to contrast the lowly status of humanity against the permanence and exulted dignity of God and the angels (Book of Numbers 23:19, Psalm 8:4); and as a future eschatological figure whose coming will signal the end of history and the time of God's judgement (Daniel ch.7).

In the book of Daniel (composed between 167 and 164 BC. chapter 7 tells of a vision given to the prophet Daniel in which four "beasts," representing pagan nations, oppress the people of Israel until judged by God. Daniel 7:13-14 describes how the "Ancient of Days" (God) gives dominion over the earth to "one like a son of man," who is later explained later as standing for "the saints of the Most High" (7:18, 21-22) and "the people of the saints of the Most High" (7:27). The "saints" and "people of the saints" in turn probably stand for the people of Israel – the author is expressing the hope that God will take dominion over the world away from the beast-like "nations" and give it human-like Israel.

While Daniel's "son of man" probably did not stand for the Messiah, later Jewish works such as the Similitudes of Enoch and 4 Ezra consistently gave it this interpretation. The Similitudes (1 Enoch 37-71) uses Daniel 7 to produce an unparalleled messianic Son of Man, pre-existent and hidden yet ultimately revealed, functioning as judge, vindicator of righteousness, and universal ruler. The Enochic messianic figure is an individual representing a group, (the Righteous One who represents the righteous, the Elect One representing the elect), but in 4 Ezra 13 (also called 2 Esdras) he becomes an individual man.

The New Testament features the indefinite "a son of man" in Hebrews 2:6 (citing psalm 8:4), and "one like a son of man" in Revelation 1:13 and 4:14 (referencing Daniel 7:13's "one like a son of man"). The four gospels introduce a totally new definite form, the awkward and ambiguous "ὁ υἱὸς τοὺ ἀνθρώπου", literally "the man's son." In all four it is used only by Jesus (except once in the gospel of John, when the crowd asks what Jesus means by it), and functions as an emphatic equivalent of the first-person pronoun, I/me/my. Modern scholarship increasingly sees the phrase not as one genuinely used of Jesus but as a one put in his mouth by the early Church.

As for Christ

Son of God" implies his deity, "Son of Man" implies his humanity