The name of the fifth son is Peter.

Step-by-step explanation:

We are given that Peter's father has five sons. The names of four sons are Fefe, Fifi, Fafa and Fufu respectively.

We have to tell the name of fifth son.

Since in the question, it is written in the starting that Peter's father has five sons which means that one of the name of his father's son is Peter.

And the name of other four sons are already provided in the question that are: Fefe, Fifi, Fafa and Fufu

This means that the name of the fifth son is Peter.