My father was born in France in 1916 to both Polish parents.
France didn't change from
jus sanguinis to
jus soli until 1927, and then only for those born on French soil of a French mother. Unless your grandparents naturalised some time before your father turned 18, he should still be considered a Polish citizen. The fact that he served in the Polish Army during WWII should avoid any further questions if you can document it.