Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Second Cousin
If you are female and have a son, your aunt's niece's son could be you. Your aunt's niece's son could also be your first cousin, once removed, if he is the son of one of your first cousins. Your aunt also may have nieces who are not related to you, and then the son is not related to you either.
Your niece's son is your grand nephew, and you are his great aunt.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
He is their Uncle and they are nieces and nephews.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Second cousins.
Assuming he is your only sibling with female children, you would have two (2) nieces.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Your brother's children would be your nieces and nephews.
Your brother-in-law's children are your nephews and nieces.