Though the verse is literally speaking about King David, the relation of David to Christ means the verse also prefigures the preeminence of King Jesus. The throne, through David’s line, would last forever through Christ.
Gill’s Exposition says,
Also I will make him my firstborn,…. Or, “make him the firstborn”; make him great, as Jarchi interprets it; give him the blessing, the double portion of inheritance: so Christ is made most blessed for ever, and has all spiritual blessings in his hands; and is heir of all things, and his people joint-heirs with him. Christ is God’s “firstborn”, or “first begotten”, Hebrews 1:6, being begotten by him, and of him; … even him the Father promises to make “higher than the kings of the earth”; having a kingdom of a superior nature to theirs, and a more extensive and durable one; and even they themselves shall be subject to him; hence he is called “King of kings”, Revelation 19:16.
The King of Kings shall reign forever, His Kingdom shall endure. See Day 16: Two Kingdoms for related verse & info.
Thirty Days of Jesus Series-
Day 1: The Virgin shall conceive
Day 2: A shoot from Jesse
Day 3: God sent His Son in the fullness of time
Day 4: Marry her, she will bear a Son
Day 5: The Babe has arrived!
Day 6: The Glory of Jesus
Day 7: Magi seek the Child
Day 8: The Magi offer gifts & worship
Day 9: The Child Grew
Day 10: The boy Jesus at the Temple
Day 11: He was Obedient!
Day 12: The Son!
Day 13: God is pleased with His Son
Day 14: Propitiation
Day 15: The gift of eternal life
Day 16: Two Kingdoms
Day 17: Jesus’ Preeminence