"Son of Man" was actually a title. You see in the OT the Jews were expecting, well, three different people to come and play a part in the redemption of Israel. That's why they were so confused at Jesus' claims and couldn't believe who He was....why the disciples themselves we so slow to realisation!
The Messiah (also known as the Christ, the Anointed One) was going to be a mighty King, a descendant of David, who would through might free them from enslavement. He would be human, but a greater king than David even! Then there was the 'suffering servant', not talked about a lot in the OT, but seen in Isaiah 53. It talks of how this man would be humble and passive, how he will suffer and die for the transgressions of many. It was not generally perceived that the suffering servant was connected with the Messiah, but of course in hindsight we know better!
The third person is the "Son of Man" who we see in Daniel. The Son of Man was considered to be a heavenly being, all powerful, who would come. In Daniel 7:13-14 it says:
[The Son of Man Is Given Dominion]
[13] “I saw in the night visions,
and behold, with the clouds of heaven
there came one like a son of man,
and he came to the Ancient of Days
and was presented before him.
[14] And to him was given dominion
and glory and a kingdom,
that all peoples, nations, and languages
should serve him;
his dominion is an everlasting dominion,
which shall not pass away,
and his kingdom one
that shall not be destroyed.
(Daniel 7:13-14 ESV)
Many circles in Judaism interpreted this figure in individualistic terms. The Son of Man became a Heavenly figure who had been preserved in the presence of God. In God's time he would come to earth to raise the dead, to judge the wicked, to redeem God's people and gather them into a glorious, everlasting Kingdom.
It's easy for us to see how all these figure are Christ! But back in Jesus' time the Jews expected them not to be one and the same.