Behold, I have given him for a witness. By ordinary rules of grammar, the pronoun "him" should refer to David; and so the passage is understood by Gesenius, Maurer, Hitzig, Ewald, Knobel, Delitzsch, and Mr.
Cheyne. But, as Isaiah frequently sets aside ordinary grammatical rules, and as the position to the person here spoken of seems too high for the historical David, a large number of commentators, including Vitringa, Michaelis, Dathe, Rosenmuller, Umbreit, and Dr.
Kay, consider that the Messiah is intended. It is certainly difficult to see how the historical David could be, at this time and in the future, a "leader and commander to the peoples" who were about to flock into the Messianic kingdom.
A witness … a leader and commander. Christ was all these. He "came to bear witness to the truth" (John 18:37), and "before Pilate witnessed a good confession" (1 Timothy 6:13). He "feeds and leads" his people (Revelation 7:17), and is the "Commander" under whose banner they serve (2 Timothy 2:3, 2 Timothy 2:4).
What he is to his people, he is also of the "peoples" generally; for they have been called into his kingdom, People … people; rather, peoples.