your second cousin or nephew or niece
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
You and your first cousin's son are first cousins, once removed.
Your cousins son will be your second cousin xx
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.
Your daughter and your first cousin's son are second cousins to each other.
Yes, of course your son and your great niece are related. They are first cousins, once removed, to each other. (Your great niece's parent, your niece or nephew, and your son are first cousins to each other.)
Your daughter and your nephew are first cousins to each other. Your daughter and your nephew's son are first cousins, once removed, to each other.
Your son and your nephew are first cousins to each other. Your son's daughter and your nephew are first cousins once removed.
Your son and your aunt's child are first cousins, so your son and your aunts grandson (the son of your aunt's child) are first cousins, once removed.
Your daughter and your sister's son are first cousins to each other.
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
You and your mother's brother are first cousins. There is a generation difference between your son and the daughter. That makes them first cousins once removed.