There is no proper term for this particular familial place. I would suggest just his name or your step-father's 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....
Your grandson.
Grandson :)
Your sisters grandson would be your grandnephew.
First cousins twice removed.
You do you call your sons's son your "grandson" !
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.
The son of a father's son is commonly referred to as a grandson.