As a younger Christian I was much puzzled by this term. Not helped by the poor Christian education provided by my church, of course.
One minister, RS, offered a suggestion that it had something to do with the Son of Man in Daniel 7:13; and this has some weight, I think.
Perhaps it was merely the human-like form of the vision, that attracted the title, but even here, it seems to denote something of greater significance, a sort of summative title.
The son is usually the inheritor, so what does the 'son of man' inherit?
I think it might be that, as the incarnate (to be, in Daniel) God...the Son, he inherits all that man as created was to be, pre-fall. He also, as the Lamb of God, inherits the consequence of the fall for man as created. This bears out both his conduct on earth pre-resurrection, but as the divine adopting human form, and his accepting the punishment given for sin (the repudiation of God-ness, and of God himself): death, despite him not meriting it. He accepted it to defeat it.