He is either you grandfather or first cousin twice removed.
Your grandfather's nephew is your parent's first cousin and your first cousin, once removed.
You share a common ancestor. His nephew would be first cousins to one of your parents. So you would be first cousins once removed.
First cousin twice removed.
Your grandfather's great nephew is your second cousin.
yes
Your nephew's son is your great nephew.
cousin
Yes, your grandfather's granddaughter could be your sister, making her son your nephew.
3rd cousin
Your great great grandfather.
What is your great-great-great grand nephew?!
The child of your nephew is your great nephew (if a son) or your great niece (if a daughter).
Your nephew's daughter's son is your great-great nephew. Your nephew's daughter is your great niece.
Your nephew.
Nephew