The son of your great niece is your great great nephew. You are his great great uncle.
Your niece's son is your great nephew.He would be your grandnephew
The son of your great niece is your great great nephew.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your niece's son's daughter is your great grand niece. Your niece's son is your grand nephew or your great nephew, whichever term you prefer.
The son of your great niece is your great great nephew. You are his great great uncle.
Your husband is great uncle to your niece's son (your great nephew).
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).
A niece's son calls his uncle his uncle, just like everyone else.The niece's son calls her uncle his great uncle. He is the great uncle's great nephew.
Your niece's son is your great-nephew. Your niece's grandson is your great-great nephew.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
That's easy cousins.