OP: Where does the OT predict the Son of man?
see EDIT TO ADD below
And there were certain Greeks among them that came up to worship at the feast: The same came therefore to Philip, which was of Bethsaida of Galilee, and desired him, saying, Sir, we would see Jesus. Philip cometh and telleth Andrew: and again Andrew and Philip tell Jesus. And Jesus answered them, saying, The hour is come, that the Son of man should be glorified. John 12:20-22
So, with these verses, we have Jesus identifying Himself as the Son of man and the idea of glorification. What does glorification mean in this context?
Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. John 12:24
Jesus speaks of death (fall into ground and die) and resurrection (bring forth much fruit).
But the people do not comprehend the idea that Messiah would come, die, resurrect. They only understand that Messiah would come and rule, wresting control away from Rome.
The people answered him, We have heard out of the law that Christ abideth for ever: and how sayest thou, The Son of man must be lifted up? who is this Son of man? John 12:34
So, where is this two-pronged idea? Who is the Son of man? Christ points to Isaiah's prophecy about Himself.
That the saying of Esaias the prophet might be fulfilled, which he spake, Lord, who hath believed our report? and to whom hath the arm of the Lord been revealed? John 12:38
What does Isaiah say as regards the Son of man to which Jesus Son of man spoke?
Who hath believed our report? and to whom is the arm of the LORD revealed? Isaiah 53:1
For he shall grow up before him as a tender plant, and as a root out of a dry ground: he hath no form nor comeliness; and when we shall see him, there is no beauty that we should desire him. He is despised and rejected of men; a man of sorrows, and acquainted with grief: and we hid as it were our faces from him; he was despised, and we esteemed him not. Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted. But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed. All we like sheep have gone astray; we have turned every one to his own way; and the LORD hath laid on him the iniquity of us all. He was oppressed, and he was afflicted, yet he opened not his mouth: he is brought as a lamb to the slaughter, and as a sheep before her shearers is dumb, so he openeth not his mouth. He was taken from prison and from judgment: and who shall declare his generation? for he was cut off out of the land of the living: for the transgression of my people was he stricken. And he made his grave with the wicked, and with the rich in his death; because he had done no violence, neither was any deceit in his mouth. Yet it pleased the LORD to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the LORD shall prosper in his hand. He shall see of the travail of his soul, and shall be satisfied: by his knowledge shall my righteous servant justify many; for he shall bear their iniquities. Therefore will I divide him a portion with the great, and he shall divide the spoil with the strong; because he hath poured out his soul unto death: and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors. Isaiah 53:2-12
The well-known Messiah as suffering servant verse.
Jesus identifies as the Son of man in the context of Messiah as suffering servant who dies as an offering for our sins, yet who is resurrected for our lives.
EDIT TO ADD
The specific verse in Isaiah 53:2 says,
For he shall grow up before him as a tender plant, and as a root out of a dry ground: he hath no form nor comeliness; and when we shall see him, there is no beauty that we should desire him.
In turn, this prophecy of a "tender plant" and "as a root out of dry ground" refers to, among other places, Isaiah 11:1.
And there shall come forth a rod out of the stem of Jesse, and a Branch shall grow out of his roots:
This is the specific reference to Son of man in the Old Testament to which Christ referred in the New Testament. Jesus was the Branch, the rod, out from the stem of a man Jesse, who was the father of David.
Incidentally, this Son of man from the root of David is shown in Rev. 5:5
And one of the elders saith unto me, Weep not: behold, the Lion of the tribe of Juda, the Root of David, hath prevailed to open the book, and to loose the seven seals thereof.
Lastly, this reference helps explain "He will be called a Nazarene" in Matthew 2:23.
that it might be fulfilled which was spoken by the prophets, He shall be called a Nazarene--better, perhaps, "Nazarene." The best explanation of the origin of this name appears to be that which traces it to the word netzer in Isa 11:1 --the small twig, sprout, or sucker, which the prophet there says, "shall come forth from the stem (or rather, 'stump') of Jesse, the branch which should fructify from his roots." The little town of Nazareth, mentioned neither in the Old Testament nor in JOSEPHUS, was probably so called from its insignificance: a weak twig in contrast to a stately tree; and a special contempt seemed to rest upon it--"Can any good thing come out of Nazareth?" ( Jhn 1:46 ) --over and above the general contempt in which all Galilee was held, from the number of Gentiles that settled in the upper territories of it, and, in the estimation of the Jews, debased it. Thus, in the providential arrangement by which our Lord was brought up at the insignificant and opprobrious town called Nazareth, there was involved, first, a local humiliation; next, an allusion to Isaiah's prediction of His lowly, twig-like upspringing from the branchless, dried-up stump of Jesse; and yet further, a standing memorial of that humiliation which "the prophets," in a number of the most striking predictions, had attached to the Messiah.
Jamieson, Fausset & Brown