There is no proper term for this particular familial place. I would suggest just his name or your step-father's grandson.
it is your nephew if its your bother/sisters child or it would be your child
Your father's brother is your uncle. Your uncle's son is your first cousin. Your first cousin's son would be your first cousin once removed.
Your grandfather's nephew's son is your grandfather's great nephew. He is also your great aunt's or great uncle's grandson, and your second cousin.
your great nephew?
great nephew
a grandson
Your grandson.
Your father's sister is your aunt. Her son is your first cousin. Her grandson is your first cousin, once removed.
grandson....
Grandson :)
Your grandson.
First cousins twice removed.
Your sisters grandson would be your grandnephew.
Your grandson.
You do you call your sons's son your "grandson" !
You speak to her about it or you ask someone who she respects to do it for you.
Your granddaughter and your sister's grandson are second cousins.
you call your parents sisters aunty