Your first cousin is your son's first cousin, once removed.
The name given to the son of your aunt is first cousin or simply cousin.
Your first cousin's grandchild is your first cousin twice removed
Your first cousin once removed.
Your first cousin is your son's first cousin, once removed. Since that cousin's sister has the same parents, her relationship to your son is the same - first cousin, once removed.
Your cousin's son is your cousin, once removed, but it is probably best to call him or her by their first name.
Your mother's first cousin's son is your second cousin. Your mother's second cousin's son is your third cousin. And so forth.
I'd normally call him by his first name. And the son of your aunt is your first cousin.
Your father's niece is your first cousin. Her son is your first cousin, once removed.
Your father's niece is your first cousin. Her son is your first cousin, once removed.
Your first cousin.
Your first cousin.
The son of your grandfather's First Cousin is your parent's Second Cousin and your Second Cousin, once removed. The grandson of your grandfather's First Cousin is your parent's Second Cousin, once removed, and your Third Cousin.