“authority” = exousia = the power of choice; the liberty of doing as one pleases.

“because he is the Son of man” this is the second reason given by Jesus for the Father to have given judgment to the Son.  The first was given in verse 23 “that all men should honor the Son, even as they honor the Father” which is speaking of worship.

There is a double reason for the Son being made the Judge based upon the fact that He is the Son of Man:

1. sharing our humanity. The Son is a judge who shares the nature of the judged.  McGarvey points out that:
   “We can see several reasons, humanly speaking, why the humanity of Jesus should be made a ground for committing the judgment of the races of men to him:
   1. Jesus having experienced our infirmities and temptations, we can feel sure of his sympathy. #Heb 4:15,16
      Heb 4:15-16  For we do not have a high priest who is unable to sympathise with our weaknesses, but we have one who has been tempted in every way, just as we are—yet was without sin. Let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need.
   2. Jesus, partaking of the nature of both God and man, is, because of his unique nature, the only fit daysman or umpire between them. #Job 9:33 Possibly we may regard it as a reward of humility.” #Php 2:8,9 (McGarvey TFG 203-204)
      Job 9:33  If only there were someone to arbitrate between us, to lay his hand upon us both,
2. He is the “Son of man” in Daniel who receives “dominion, and glory, and a kingdom” which would qualify Him as a judge. #Dan 7:13-14