Your great great nephew's child is your great great great niece or great great great nephew, depending on the sex of the child.
Your great aunt's child's so is her grandson and your second cousin.
The child of your great nephew (or grand nephew), would be your great great nephew (or great grand nephew).
He would call your aunt his great aunt. In informal situations, great aunts are sometimes called, simply aunt.
He would call your aunt his great aunt. In informal situations, great aunts are sometimes called, simply aunt.
great nephew
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.
your cousin
Your great aunt's son is your parent's first cousin and your first your cousin, once removed.
your great nephew
Great great nephew
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your aunt's son is your first cousin, once removed. Your great aunt's son and one of your parents are first cousins. You are one generation removed from them so you and your great aunt's son are first cousins, once removed.
A nephew is a cousin to his aunt's son.
Your son and your aunt's son are first cousins once removed to each other.