What is «أَبُو» in «جَاءَ الَّذِي قَامَ أَبُوهُ»?

Relative Nouns — Ṣilah, ʿĀʾid, Forms and Syntax

• A nominative subject marked by wāw
• An accusative mubtadaʾ
• A direct object
• A predicate of kāna