you would be the child's great aunty or uncle Therefore, they would be your great niece or nephew.
You would be his great aunt. To you he would be your great nephew.
Your nephew's son is your grand nephew or great nephew. Some people say one, some say the other.
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
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 nephew's child is your great nephew or great niece depending upon sex.Your nephew's grandchild is your great great nephew or great great niece, depending on sex.
Your great nephew's child is your great great niece (if a girl) or great great nephew (if a boy).
Your nephew's daughter's son is your great-great nephew. Your nephew's daughter is your great niece.
Your nephew's daughter's son is your great grand nephew (or great great nephew). You are his great great aunt if you are a woman, or his great great uncle if you are a man.
Your great nephew's child is either your great-great niece or your great-great nephew, depending.
Your nephew's daughter is your great niece. Her child is your great great niece or great great nephew.