نَهَارُهُ صَائِمٌ
His day is fasting; that is, he fasts during the day.
A person—not a period of daylight—performs a fast. The participle ṣāʾim (صائم) is nevertheless assigned to nahār (نهار), the time in which fasting occurs.
Compare “الرَّجُلُ صَائِمٌ” (the man is fasting), where the proper bearer is named. The revised subject identifies a temporal association and changes the predicational relation.