Your first cousin's son is your first cousin, once removed.
Your second cousin's son is your second cousin, once removed.
Your sister's daughter and your son are first cousins to each other.
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 granddaughter and your niece's son are second cousins.
Your son's child and your nephew 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.
The relationship to the grandfather or the relationship to the grandson of the grandfather? Let's say your name is Ken, your grandfather's name is William, his brother's name is Eli, and Eli's son is named John. If you're asking about William and John's relationship, they are first cousins. If you're asking about Ken and John's relationship, they are first cousins once-removed.
First cousins
First cousins