He is either you grandfather or first cousin twice removed.
Your grandfather's great nephew is your second cousin.
yes
Yes, your grandfather's granddaughter could be your sister, making her son your nephew.
Your nephew's son is your great nephew.
cousin
3rd cousin
What is your great-great-great grand nephew?!
Your great great grandfather.
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.
Nephew
Your nephew.