A cousin or a first cousin.
nees
Your uncle's son is your first cousin.
It means you have (or had) at least two uncles and at least two great-uncles, and that is all it means.
your uncles son is your cousin
His great uncle
Your cousin in law
great uncle
They are your cousins.
He could be your brother. He could be a first cousin. If the uncle married into the family (Married the sister of one of your parents), the brother would not be related in any way and neither would his son.
your cousin
Your mother's brother is your uncle.The children of your aunts and uncles are your first cousins.
Your uncle/aunt's son/daughter is an adult and you are still small, your uncle/aunt's daughter had a child, and that child will call you uncle. Uncles/aunts are your dad/mom's siblings.
Your aunts and uncles, and one of them is your father or mother.