Cambyses.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
Your great, great great grandson.
The son of your great niece is your great great nephew. You are his great great uncle.
The son of your great niece is your great great nephew.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.
your son will be grandfather of your great grand son. :)
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
The son of your great niece is your great great nephew. You are his great great uncle.
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.