A person's nephew's son would be the person's grand nephew. "Grand" is added to many genealogy relationships as a way to show the addition of one more generation. For example, a person's daughter's child would be referred to as the person's grandchild.
It is common for people to misunderstand the difference between the prefixes "great" and "grand" being added to a family relationship classification. "Grand" is always added to show one generation after the primary relationship. "Great" is then used along with "grand" to note a relationship two generations down the line. So a person's grand nephew's son would be that person's great grand nephew.