The son of your great grandfather's brother was your grandfather's first cousin. That makes him your first cousin, twice removed.
Your grandson.
The son of a father's son is commonly referred to as a grandson.
You do you call your sons's son your "grandson" !
Grandson :)
Your grandson.
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 grandson.
Your brother's grandson is your great nephew.
Your dad would typically refer to your son as his grandson.
A person's son's son would be their grandson. Your son's daughter would be your granddaughter. Your grandson's son would be your great grandson.
A person's son's son would be their grandson. Your son's daughter would be your granddaughter. Your grandson's son would be your great grandson.
you can call him your nephew if you are mom's daughter's sibling or if you will be the mom you can call him your grandson.