Klaus Koch writes that the "Son of Man" is probably the archangel Michael in the Old Greek version of Daniel 7.
Klaus Koch, "Stages in the Canonization of the Book of Daniel," in The Book of Daniel: Composition and Reception, ed. John J. Collins and Peter W. Flint, 2 vols. (Supplements to Vetus Testamentum LXXXIII,II; Formation and Interpretation of Old Testament Literature II, 2; Leiden: Brill, 2001), 2:426-27
The message of the Old Greek version differs in some important respects from the Aramaic-Hebrew version . . .Whereas in the Vorlage the eschatological Son of Man probably denotes the archangel Michael who is authorized for the coming kingdom of God by the Ancient of Days (See Collins [Daniel, 304-10]), the Old Greek identifies the coming savior with the Ancient of Days himself and apparently interprets him as the Messiah, "whose origin is from old, from ancient days" (Micha 5:2). As a consequence of this, the holy ones of chapter 7, originally angels, (Collins, Daniel, 313-17) were seen as "the holy people of the Most High" on earth (7:27).