You son and your niece are first cousins. Your son and your niece's son are first cousins, once removed.
Your niece's son is your great nephew.He would be your grandnephew
Your son and your niece are first cousins.
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.
Your granddaughter and your niece's son are second cousins.
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
Your niece's son and your daughter are first cousins, once removed.
The son of your great niece is your great great nephew.
Your husband is great uncle to your niece's son (your great nephew).
Your aunt's son and her niece are first cousins to each other.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.