Introducing a boy, a girl said, he is the son of the father of my uncle. How is the boy related to the girl

The boy is the girl's uncle.

Explanation:

The girl's uncle is her father's or mother's brother. This means that the uncle's father is this girl's grandfather, since he is the father of her father or her mother. Therefore, all of her grandfather's children are siblings of her father or mother. Therefore, all her father's or mother's brothers are her uncles.

In this case, if the boy is the son of her uncle's father, it means that the boy is her uncle's brother, and therefore the boy is her uncle, the son of her grandfather.