He is your first cousin once removed.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.
great nephew
I believe they would be great aunts or great uncles.
Your mother's aunt's son is her first cousin and your first cousin, once removed.
Your son and your aunt's child are first cousins, so your son and your aunts grandson (the son of your aunt's child) are first cousins, once removed.
The grandchildren of any of your great uncles and great aunts are your second cousins.
Your grandmother's sister is your great aunt, her grand children are your second cousins twice removed. Genetically speaking this is barely a relationship - but if you were raised together as cousins the familiar bond can be tight.
your cousin
The possessive form for the plural noun great aunts is great aunts'.Example: My three great aunts' house is a great place to visit.
Your great aunt's son is your parent's first cousin and your first your cousin, once removed.
Great uncle / Great nephew
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)