I am going through my late grandfathers documents from WW2 and I discovered he has a German Identity Card of him in the Hitler youth!!!!!!
In October 1939 Polish lands occupied by Germany were divided into General Government (German administration of occupied territories) and the western regions, which were annexed directly into the 3rd Reich. There were much different policies applied to those territories. Majority of annexed lands were a part of Germany before 1918, Poles living there often spoke German and many had some German connections. Germans put a lot of pressure on such people to sign the Volksliste and become a kind of semi-Germans (those who refused were at best deported into General Government or send to proper Germany as forced laborers), in some cases (but that mainly in Silesia and Kashubia) people were classified as such without their will.
There are many possibilities... Your family could have had some German connections, significiant enough to get into big troubles If they resisted Germanization or... they decided to collaborate without much pressure... or children were taken away from parents nad put into German families, It didn't happen only to infants... What I don't understand here is that he was classified as Polish in the Nazi documents, the whole idea of the Volkliste was turning such people into Germans... Maybe, as someone else said, It was late at war when Krauts were desperately looking for people to throw granades at Soviet tanks ?