Depends on who is discussing him. He would be your first cousin.
Your mother's brother is your uncle. Your uncle is your son's great uncle. You are the nephew of your mother's brother. Your son is his great 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).
The son of your Uncle's son is your uncle's grandson. Your uncle's grandson is your first cousin, once removed.
Your nephew's son is your great nephew and you are his great aunt or great uncle.
Your son is your uncle's grandnephew. Your uncle is your son's great uncle.
Your sister's son is your nephew. Your uncle is your nephew's grand uncle or great uncle.
Your Uncle's son is your first cousin.
Your mother's brother's son would be her nephew and your cousin.
Your brother's (or your sister's) son is your nephew and you are his aunt if you are a female or his uncle if you are a male.
Your Uncle's son is your first cousin.
Your uncle's son is your (first) cousin.
A niece's son calls his uncle his uncle, just like everyone else.The niece's son calls her uncle his great uncle. He is the great uncle's great nephew.