The voice - the same divine herald as in Isa 40:3.
he - one of those ministers or prophets (see on Isa 40:1) whose duty it was, by direction of “the voice,” to “comfort the Lord’s afflicted people with the promises of brighter days.”
All flesh is grass - The connection is, “All human things, however goodly, are transitory: God’s promises alone steadfast” (Isa 40:8, Isa 40:15, Isa 40:17, Isa 40:23, Isa 40:24); this contrast was already suggested in Isa 40:5, “All flesh ... the mouth of the Lord.” 1Pe 1:24, 1Pe 1:25 applies this passage distinctly to the gospel word of Messiah (compare Joh 12:24; Jam 1:10).