Scripture does not refer to Christ as "the Son" of God the Father (except in prophecy—see Psalm 2:7; II Samuel 7:14) until His human birth (Hebrews 5:5-8; Luke 1:32). Likewise, the Bible does not refer to God the Father as such before that time. In fact, the Father's existence is not clearly revealed until after Christ's coming (Luke 10:22; Matthew 11:27; John 1:18).
In Old Testament times, Christ had many different titles. Since that time, the title and attributes of "Son of Man" have been added to them. This title identifies His humanity and His affinity to the rest of mankind, for we are all "sons of man." However, unlike us, He was and is also God, so He has become the perfect Mediator between God and man (Hebrews 8:6; 9:15).
To become human, He was required to divest Himself of His glory to become our sacrifice for sin (Philippians 2:5-9; Hebrews 2:9), a vital part of God's plan of salvation. The author of Hebrews writes:
Inasmuch then as the children have partaken of flesh and blood, He Himself likewise shared in the same, that through death He might destroy him who had the power of death, that is, the devil, and release those who through fear of death were all their lifetime subject to bondage. . . . Therefore, in all things He had to be made like His brethren, that He might be a merciful and faithful High Priest in things pertaining to God, to make propitiation for the sins of the people. For in that He Himself has suffered, being tempted, He is able to aid those who are tempted. (Hebrews 2:14-15, 17-18; see 4:15)
So, this title designates a very important attribute of Christ—that He was human just like us and experienced life like us. In His present position of High Priest at the right hand of God, He acts as our Advocate before the Father, helping Him to understand our weaknesses and procure mercy and aid (I John 2:1; Hebrews 2:16; 4:16; 6:19-20; 7:24-27; 10:12-22).