The phrase “Son of Man” and the different senses in which it is used is one of the most enigmatic things to me about this area. It’s very strange. Sometimes, it seems to mean humanity. Sometimes, it seems to mean a cosmic judge. Sometimes, the character Jesus uses these words to refer to himself. 

What’s the difference between the Messiah and the son of man? To understand these terms, I think we have go back to the Hebrew scriptures and look at the different contexts in which the terms are used and the different sets of expectations concerning these figures. I don’t have enough background to have references at the ready, but I’m pretty sure that the messiah was pretty much always (until the Christians came along) expected to be an earthly, human person who would bring great honor and glory to the Jewish people in some way. Often, it was expected that the Messiah would be a future king. Someone who would rule over a restored Israel. Or possibly a great religious teacher or lawgiver.

The Son of Man, when used with the definite article, particularly as it’s used in the Book of Daniel, appears to refer to a (humanlike?) cosmic divine agent who arrive at the end of time to judge the earth and separate the righteous from the damned. 

Your suggestion that the son of man and the messiah could be the same figure is interesting, but I get the sense that there were slightly different sets of expectations around these figures. The early Christians want to say that Jesus is both Messiah and Son of Man. I’m not sure that the Jewish tradition would say that the Messiah and the Son of Man are the same thing, necessarily, and they certainly would have scoffed (and did) at the notion that this Jesus of Nazareth could have been either.