who is Son of man,?

A question of Christians:

in Your Bible

85 Times the name of the Son of man is come.

and

In one case

Jesus (PBUH) says,

Me with the Son of Man will return

so

who is Son of man,?

Some of you Clergymen says:

he is Holy Spirit

( Is son of man a Angel )

And a few say he is Jesus Christ

( Is it possible that Jesus with his self Return )

so 

who is Son of man,?

Muslims say that our Savior

(Imam Mahdi (pbuh) and Jesus Christ (PBUH))

Together returns

Is it the Son of Man as Imam Mahdi (pbuh) is

So we can say

Muslims and Christians expecting an appearance

And wait for the Savior of the same

So why Muslims and Christians don't work together for the emergence of

In the hope of peace all over the world and the emergence of the two great