The Lord is called the Son of Humanity when the subject is redemption, salvation, reformation, and regeneration, as we can tell from the following:
The Son of Humanity came to give his life as a redemption for many. (Matthew 20:28; Mark 10:45)
The Son of Humanity has come to save and not to destroy. (Matthew 18:11; Luke 9:56)
The Son of Humanity has come to seek and to save that which was lost. (Luke 19:10)
The Son of Humanity came so that the world would be saved through him. (John 3:17)
The one who sows good seed is the Son of Humanity. (Matthew 13:37)
Here the subject is redemption and salvation; and since they are effected by the Lord through the Word, he refers to himself as the Son of Humanity.
The Lord said that the Son of Humanity has power to forgive sins (Mark 2:10; Luke 5:24)—that is, power to save. He also said that he was Lord of the Sabbath because he was the Son of Humanity (Matthew 12:8; Mark 2:28; Luke 6:5)—because he himself is the very Word that he is then teaching.
He also says in John,
Do not work for the food that perishes, but for the food that endures to eternal life, which the Son of Humanity will give you. (John 6:27)
Food means everything true and good in the teaching drawn from the Word and therefore from the Lord. This is also the meaning of the manna and of the bread that comes down from heaven, as well as the meaning of these words in the same chapter:
Unless you eat the flesh of the Son of Humanity and drink his blood, you will not have life within you. (John 6:53)
The flesh or bread is good actions done from love as a result of the Word, and the blood or wine is good actions done from faith as a result of the Word, both of which come from the Lord.
“The Son of Humanity” means much the same in various other passages where it is found, such as the following:
Foxes have dens and birds have nests, but the Son of Humanity has nowhere to lay his head. (Matthew 8:20; Luke 9:58)
This means that the Word had no place among the Jews, as the Lord also says in John 8:37, and that the Word was not abiding in them, because they did not acknowledge him (John 5:38).
The Son of Humanity means the Lord as the Word in Revelation as well:
In the midst of seven lampstands I saw one like the Son of Humanity, clothed with a garment down to the feet and girded about the chest with a golden band. (Revelation 1:13 and following)
In this passage various images are used to represent the Lord as the Word, so he is also called “the Son of Humanity.” In David:
Let your hand be with the man of your right hand, with the Son of Humanity whom you have strengthened for yourself. Then we will not turn back from you. Bring us to life. (Psalms 80:17, 18, 19)
The man of your right hand in this passage is also the Lord as the Word, and so is the Son of Humanity. He is called “the man of your right hand” because the Lord has power from divine truth, which is also the Word; and he gained divine power when he fulfilled the whole Word. That is why he also said that they would see the Son of Humanity sitting at the right hand of the Father with power (Mark 14:62).
The reason “the Son of Humanity” means the Lord as the Word was that the prophets were also called “children of humanity.” The reason they were called this is that they represented the Lord as the Word and therefore meant the teaching of the church drawn from the Word. That is exactly what people in heaven understand by “prophets” when they are mentioned in the Word. The spiritual meaning of “prophet” and also of “son of humanity” is the teaching of the church drawn from the Word, and when it is said of the Lord, it means the Word itself.
For the prophet Daniel being called a son of humanity, see Daniel 8:17.
For the prophet Ezekiel being called a son of humanity, see Ezekiel 2:1, 3, 6, 8; 3:1, 3, 4, 10, 17, 25; 4:1, 16; 5:1; 6:2; 7:2; 8:5, 6, 8, 12, 15; 11:2, 4, 15; 12:2, 3, 9, 18, 27; 13:2, 17; 14:3, 13; 15:2; 16:2; 17:2; 20:3, 4, 27, 46; 21:2, 6, 9, 12, 14, 19, 28; 22:18, 24; 23:2, 36; 24:2, 16, 25; 25:2; 26:2; 27:2; 28:2, 12, 21; 29:2, 18; 30:2, 21; 31:2; 32:2, 18; 33:2, 7, 10, 12, 24, 30; 34:2; 35:2; 36:1, 17; 37:3, 9, 11, 16; 38:2, 14; 39:1, 17; 40:4; 43:7, 10, 18; 44:5.
We can see from this that the Lord as the Divine-Human One is called “the Son of God,” and as the Word is called “the Son of Humanity.”
from The Lord, Sections 27, 28