Compare the reason, not just the visible particle:
يَا إِبْرَاهِيمُ - already definite because إِبْرَاهِيمُ is a proper name.
يَا وَلَدُ - وَلَدٌ was indefinite, but direct address now points to a particular boy.
Both are vocatives, but only the second illustrates definiteness created through the taught vocative use.
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Compare the reason, not just the visible particle:
يَا إِبْرَاهِيمُ - already definite because إِبْرَاهِيمُ is a proper name.
يَا وَلَدُ - وَلَدٌ was indefinite, but direct address now points to a particular boy.
Both are vocatives, but only the second illustrates definiteness created through the taught vocative use.