He would be your nephew.
Your sister's son will call you his uncle, if you are a man, or his aunt, if you are a woman.
I'd use their name. There is no official name for it. You do not share a common ancestor, so you are not related.
You call him your nephew. (pronouced nes-few)
Shout out their name and if they here you they'll probably answer or you could call them on their phone
Your sister's son is your nephew. You are his aunt or uncle.
If your sister had a son, then he is your nephew.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your father's sister's son (or daughter) is your first cousin.
He would call her his aunt.
Newphew
Your nephew.