He would be your second cousin.
Your father's first cousin's son is your second cousin. Your father's second cousin's son is your third cousin. If your are asking about your fathers cousins who are in a removed cousin relationship to your father, their relationship to you be more complex, with two different possibilities in each case.
You are each others cousins.
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
Second cousins once removed.
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.
Cousine