Your grandson and your first cousin are first cousins, twice removed.
Your first cousin's grandson is your first cousin twice removed.
Your grandmother's first cousin's grandson is your third cousin.
Your first cousin's grandson son is your grandson's third cousin. Your second cousin's grandson is your grandson's fourth cousin. Etc., etc.
The grandson of your father's first cousin is your second cousin once removed.
Your great grandmother's first cousin's grandson is your third cousin once removed
Your grandson and your first cousin are first cousins, twice removed. Your grandson and your second cousin are second cousins, twice removed. and so forth.
Your first cousin's grandson is your first cousin twice removed. Your second cousin's grandson is your second cousin twice removed. Your third cousin's grandson is your third cousin twice removed. And so forth. If your cousin is once removed, twice removed, etc., the relationship possibilities for you and the grandson become more varied and complex.
Your aunt's grandson is the son of your First Cousin and is therefore your First Cousin once removed Ref: http://freepages.genealogy.rootsweb.ancestry.com/~gentutor/chart.html
Your aunt's great grandson is your first cousin twice removed.
Your first cousin's grandson is your first cousin, twice removed. Your second cousin's grandson is your second cousin, twice removed.
Your mother's first cousin's son is your second cousin. Her first cousin's son's son (her first cousin's grandson) is your second cousin, once removed.
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.