But Satan made him forget the mention [to] his master, and he remained in prison several years.”

The pronoun "him" in "made him forget" (ansahu) refers to the young man who was with our master Yusuf in the prison. He had given him the glad tidings, when interpreting his dream, that he would be released from prison and requested him to mention him to his master, the king. So the young man forgot to mention his "Lord" (rabbahu)—who is Yusuf—before the king when he returned to him as a cupbearer as he was before.