Adam Clarke on Psalms 89:27

I will make him my first-born - I will deal with him as a father by his first-born son, to whom a double portion of possessions and honors belong. First-born. is not always to be understood literally in Scripture. It often signifies simply a well-beloved, or best-beloved son; one preferred to all the rest, and distinguished by some eminent prerogative. Thus God calls Israel his son, his first-born, Exo 4:22. See also Sirach 36:12. And even Ephraim is called God's first-born, Jer 31:9. In the same sense it is sometimes applied even to Jesus Christ himself, to signify his supereminent dignity; not the eternal Sonship of his Divine nature, as inveterate prejudice and superficial thinking have supposed.