I emailed Lexmotion and said that they most likely cannot help.
I know that his birth records are in the archives in Ostrowiec, I've had confirmation of this. He arrived in the US in 1921 and my grandfather was born 2 or so months after they arrived. My great-grandfather didn't complete his military service in Poland, and was naturalized around 1927, but as far as I understand, because my grandfather turned 18 before my great-grandfather aged out of obligatory military service, he didn't lose his Polish citizenship.
The issue is they told me that because my great-grandfather emigrated before the 1921 census, it will be impossible to prove that he is a citizen. Is this really the case?
If it matters, he also seems to have lived in Osiek