زَيْدٌ: first mubtadaʾ, nominative by ḍamma.
أَبُوهُ: second mubtadaʾ, nominative by wāw under the six-nouns rule; the attached «ـهُ» is a genitive possessor pronoun.
قَائِمٌ: predicate of the second mubtadaʾ, nominative.
أَبُوهُ قَائِمٌ: nominal clause, in the nominative syntactic position of the first mubtadaʾ’s predicate.
Link: «ـهُ» refers back to Zayd.