Genealogy / Polish or not? Grandfather born in region which is now Belarussian. [10]

Im English, with a Polish grandfather.

But Im unsure whether Polish people would see him as truly Polish. To explain, my Grandad was born in a region which has been in different countries at different times. When he was born it was in Poland, he spoke Polish. He subsequently emigrated to Brazil. However, when the call when out to Polish nationals around the world to help with the war effort, he did his duty and came to England to do his part. After the war he settled here. So far, so Polish. However the region he is from, a both a few years before he was born and a few years after, became part of what today is Belarussia.

So Im confused about my Grandads identity...Pole? Belarussian?