مَرَرْتُ بِرَجُلٍ أَبُوهُ عَالِمٌ: -hu points back to rajulin.
مَرَرْتُ بِرَجُلٍ قَامَ أَبُوهُ: the same attached pronoun links the verbal clause to the man.
A sentence after a noun is not automatically an adjectival clause; an appropriate meaningful and grammatical link must exist.
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The Tawābiʿ and Naʿt: Direct and Relational Attributes
مَرَرْتُ بِرَجُلٍ أَبُوهُ عَالِمٌ: -hu points back to rajulin.
مَرَرْتُ بِرَجُلٍ قَامَ أَبُوهُ: the same attached pronoun links the verbal clause to the man.
A sentence after a noun is not automatically an adjectival clause; an appropriate meaningful and grammatical link must exist.