Your son and your third cousin are third cousins, once removed, to each other.
Your son and your third cousin's son are fourth cousins to each other.
Third cousin.
Your dad's second cousin's son is your third cousin.
The son of your mother's third cousin is your fourth cousin.
The son, or daughter, of your father's first cousin is your 2nd cousin. The son, or daughter, of your father's second cousins is your third cousin. The son, or daughter, of your father's third cousin is your fourth cousin. And so on.
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.
The first cousin of your dad's first cousin is also your your dad's first cousin or is not related to him or to you. The children of two first cousins are second cousins to each other. So the son of the first cousin of your dad's first cousin is either your second cousin, or is not related to you at all.
Your grandmother's third cousin is your third cousin, twice removed.
A person's relationship to the child of a cousin is the same as that person's relationship to the cousin, with one remove added. If your cousin is your first cousin, then his son is your first cousin, once removed. If your cousin is your second cousin, then his son is your second cousin, once removed. If your cousin is your first cousin, once removed, then his son is your first cousin, twice removed.
Your first cousin's nephew is the son of your first cousin's sister or brother. That sister or brother is also your first cousin, and the son is your first cousin, once removed. So your first cousin's nephew is your first cousin, once removed. Similarly, your second cousin's nephew is your second cousin, once removed.
Your first cousin's daughter and your son are second cousins to each other. Your second cousin's daughter and your son are third cousins to each other. Your third cousin's daughter and your son are fourth cousins to each other. And so forth.
Your cousins son will be your second cousin xx
Step son or not regarding your mother's cousin that would make him your third cousin.