At a wedding reception, a man pointed towards the groom and claims the groom to be the brother of the son of his grandfather’s son. Who is the groom to the man?

Let's break down the description given by the man step-by-step from the inner relation outward:
1. "My grandfather's son" refers to either the man's father or the man's uncle.
2. "The son of his grandfather's son" can mean:
* If it's his father's son: It is either the man himself or his brother.
* If it's his uncle's son: It is his cousin.

3. "The brother of the son of his grandfather's son" means:
* Brother of himself or his brother $\to$ Brother.
* Brother of his cousin $\to$ Cousin.

Among the options provided (Brother, Brother-in-law, Uncle, Father), Brother is listed. Therefore, the groom is the man's brother.