cousin
You would be his Great Uncle and he would be your Great Nephew.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Second cousins once removed.
Your grandmother is first cousins with cousin. Your parent would be the second cousin of the cousin's son. That would make you second cousins once removed.
Your father's nephews and nieces are your first cousins.
You are their great aunt/uncle.
Nephews and nieces
2nd uncle I believe
Your great great grandfather.
your nephews
The sons of your son-in-law's siblings are his nephews. You are not related to them.However, your son-in-law is married to your daughter. The sons of her siblings are also your son-in-law's nephews. They are your grandsons.