Albert Barnes Commentary Psalms 89:27

"I also will make him [my] first-born, The highest of the kings of the earth." — Psalms 89:27 (ASV)

Also I will make him my first-born - He will be regarded and treated by me as the first-born son is in a family; that is, with distinguished favor and honor. Compare Genesis 27:19; Genesis 29:26; Exodus 4:22; Exodus 13:12; Jeremiah 31:9. See also the notes at Colossians 1:15, notes at Colossians 1:18.

Higher than the kings of the earth - Than other kings; the most exalted among kings and rulers. This was entirely fulfilled in David, who occupied a pre-eminence among princes and rulers which no other king did: a prominence alike in his own personal character and his reign; in his relation to God; and in the fact that he was the ancestor of the Messiah, the King of kings, and Lord of lords (Revelation 19:16); the prince of the kings of the earth (Revelation 1:5).