Let Y's son be denoted as A.
The son of Y's son means the son of A. Let this person be denoted as B.
So B is the son of A, which makes B the grandson of Y.
X is the brother of B.
Since X and B are brothers, they share the same parents.
B's father is A (who is Y's son).
Therefore, X's father is also A.
Since A is Y's son, and X is A's son, X is Y's grandson.
Therefore, X is related to Y as his Grandson.
The correct answer is Option 1: Grandson.