You're a Great Aunt to your nephew's son, who is your great nephew.
Great nephew
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
a nephew's kid to his aunt's kids would be their cousin.
Your nephew's son is your grand nephew or, some would say, your great nephew. You are his great aunt or great uncle.
He is her nephew, and she is his aunt.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
He is your nephew And you are his Aunt
Your sister-in-law in your son's aunt and he is her nephew.
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.
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
Your daughter's son is your other daughter's nephew and she is his aunt.