Someone is the answer of the fourth son.
Another possible answer is obtained from the second sentence in the question.The name of the fourth son is What. This is because, in the question the sentence "What is the name of the fourth son" does not end in a question mark but rather a full stop. This makes the answer What.
The answer is very simple. Since someone’s mom has four sons, North, West, and South, the name of the fourth son is What?
Logically we can answer the fourth son’s name will be “east” right?