Your dad's second cousin's son is your third cousin.
You are each others cousins.
He would be your second cousin.
Second cousins once removed.
They are second cousins
Your cousins son will be your second cousin xx
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.
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 niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your granddaughter and your niece's son (the grandson of one of your siblings) are second cousins to each other.
Your sister's daughter and your son are first cousins to each other. Your sister's daughter and your son's daughter are first cousins, once removed to each other. Your son's daughter and your sister's granddaughter are second cousins to each other.
The son of your second cousin is your second cousin, once removed. You are also his second cousin, once removed.
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.)