John Gill Commentary 1 Kings 11:12

"Notwithstanding in thy days I will not do it, for David thy father`s sake: but I will rend it out of the hand of thy son." — 1 Kings 11:12 (ASV)

Notwithstanding in your days I will not do it for David your
father's sake
Not for the merits of David, but the promises made to him, (2 Samuel 7:12 2 Samuel 7:13) ,

but I will rend it out of the hand of your son ;
and immediate successor, Rehoboam.