Your first cousin's sister is also your first cousin; your second cousin's sister is your second cousin, etc.
The son of your first cousin is your first cousin, once removed; of your second cousin is your second cousin, once removed; etc.
You and your first cousin's son are first cousins, once removed.
your son and your brother's son will be cousins
your son and your brother's son will be cousins
your son and your brother's son will be cousins
second cousins to his cousin's children, you'd be third cousins
Your son's child and your nephew are second cousins.
Your granddaughter and your niece's son are second cousins.
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.
First cousins
He would be your second cousin.
First cousins
Cousine