The son of your great niece is your great great nephew. You are his great great uncle.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Your niece's son is your grand nephew, and you are his great aunt.
He is your Grandfather, and is your son's Great-Grandfather.
it great nephew :)
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.
She would be your son's second cousin.
Your niece's son is your great nephew. You are his great aunt (if you are a woman) or his great uncle (if you are a man).
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Presuming your son is also your wife's son, the your wife's aunt is your son's great aunt and he is is her great nephew.
You are her grand uncle (or great aunt) and she is your grand niece.
my great grand-daughter
your uncle is your sons great uncle