Albert Barnes Commentary Psalms 89:28

"My lovingkindness will I keep for him for evermore; And my covenant shall stand fast with him." — Psalms 89:28 (ASV)

My mercy will I keep for him for evermore - I will not withdraw my favor from him, nor from his posterity (Psalms 89:33–36). In him, and in his Great Descendant, the throne shall be established forever. This dominion will not be like the changing dynasties of this world, but will be perpetual and eternal.

And my covenant shall stand fast with him - See 2 Samuel 7:14–16; 2 Samuel 23:5. It shall be firm, or established with him and his family.