This person would generally be referred to as your cousin..
Your father's brother's son (or daughter) is your first cousin, and is usually called by name.
Cousin
Your Step-brother. If he isn't your brother but he is your father's son, he's you.
cousin
your 5th cousin
Nephew
his brother
Your younger brother's wife's sister's daughter would be your son's cousin.
He is your nephew And you are his Aunt
Your father's brother's son is your first cousin and your daughter's second cousin.
The son of either your sister or your brother is your nephew. The English language does not make a distinction in that.
Depends on if you're talking about older brother, younger brother, or twin brother. And men and women use different terms. Also it also applies to certain your father's brother's son and mother's sister's son, but NOT your father's sister's son or your mother's brother's son.
You would be his uncle and he would be your nephew.
Your nephew