He would call your aunt his great aunt. In informal situations, great aunts are sometimes called, simply aunt.
I'd normally call him by his first name. And the son of your aunt is your first cousin.
The name given to the son of your aunt is first cousin or simply cousin.
He would call her his aunt.
Your aunt's son and your children are first cousins to each other.
He would call your aunt his great aunt. In informal situations, great aunts are sometimes called, simply aunt.
Your sister's son will call you his uncle, if you are a man, or his aunt, if you are a woman.
He is your cousin
Great Aunt.
You will be his aunt.
Your sister's son is your nephew. You are his aunt or uncle.
Great Aunt.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.