Introducing a boy, Amar said, "He is the son of my maternal grandfather's son." How is that boy related to Amar?

Let's break down the relation mentioned by Amar:
"My maternal grandfather's son" → Amar's maternal grandfather's son is Amar's maternal uncle (Mama).

"He is the son of [Amar's maternal uncle]" → The son of one's maternal uncle is their cousin.

Therefore, the boy is Amar's cousin.