He should have a birth certificate that shows his parents names and one of those would be either your sister or brother depending on how he supposed to be your nephew. Also the birth certificate shows place of birth so if your sister/brother lived in that area at the time it increases the likelihood that it is the correct name.
But if he does not want to deal with you, you can't force him to do so.
Your nephew's nephew is the son of one of your nephew's brothers or sisters. He is your great nephew.
Your mother's (mom) nephew is your answer.
Your nephew's son is your grand nephew or great nephew.
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
Your wife's nephew is your nephew and his son is your wife's, and your, great nephew.
The male counterpart of a niece is a nephew.
Niece would be an opposite as a niece is female and a nephew is male.
Your nephew's son is your great nephew.
Your nephew's son is your great nephew.
In English language terms for family relationships, there is no relationship called "nephew once removed." The son of your nephew is your great nephew.
Your nephew's son is your grand nephew or great nephew. Some people say one, some say the other.
My nephew cannot hold a job.I have only one nephew. Though my nephew, we are nothing alike.