That would be his nephew.
He is your grandson. If his father is your son, he is your grandson.
Your sister's son, is your nephew.
Your sister's son, is your nephew.
Your sister's son is called your nephew. You are his aunt (if you are a woman) or his uncle (if you are a man).
Your sister-in-law's grandson is your great nephew.
Your father's sister is you aunt. Her son is your first cousin. Her son's daughter (her granddaughter) is your first cousin, once removed.
boogey mans son boogey mans son
The son of either your sister or your brother is your nephew. The English language does not make a distinction in that.
Your cousin
Your cousin.
Your Nephew
cousin