The relationship sequence:
Your nephew's son is your great nephew.
Your nephew's daughter's son is your great-great nephew. Your nephew's daughter is your great niece.
The child of your nephew is your great nephew (if a son) or your great niece (if a daughter).
Your brother's son is your nephew. Therefore, your brother's son’s son would be your great-nephew. In summary, your brother's son son is your great-nephew.
great nephew
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
Your nephew's child is your great nephew (if a boy) or great niece (if a girl). You are his or her great aunt.
Your husband is great uncle to your niece's son (your great nephew).
You would be a great uncle, so they would be your great nephew
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
You would be his Great Uncle and he would be your Great Nephew.