Your niece's son is your great nephew. You are his great aunt or great uncle.
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 granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Second Cousin
Your niece's son is your grand nephew, and you are his great aunt.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
They are not related.
They are your nieces and nephews.
Some nieces of sisters are related to each other and some are not. Your niece is the daughter of your brother or sister (your siblings). If you have more than two siblings and at least two of them have daughters, those daughters are your nieces and they are first cousins to each other. You can also have a niece who is the daughter of a sibling of your spouse. If two sisters have nieces who are daughters of their spouse's siblings, those nieces are not related to each other (unless the spouses are related to each other, as when two brothers marry two sisters)
Assuming he is your only sibling with female children, you would have two (2) nieces.
You are not related to your niece's husband, although some would call him your nephew by marriage. But that is a relatively recent construction and not used by many. You could also call him your brother's (or sister's) son-in-law.
If your great uncle married into your family some of his nieces and nephews are your aunts or uncles (or your parent) and some are not related to you at all.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.