He would be your grand nephew; you would be his great-aunt.
He would be your grand nephew; you would be his great-aunt.
Grand nephew would be the term, you would be their great aunt/uncle.
You call them your nieces, or by their names.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Second Cousin
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Your niece's son is your grand nephew, and you are his great aunt.
Nieces.
If it is a boy then it is your nephew. If it is a girl then it is your niece.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your brother's daughters are your nieces
Your brother's daughters are your nieces
Assuming he is your only sibling with female children, you would have two (2) nieces.