Auslander by Paul Dowswell
When Peter's parents are killed, he is sent to an orphanage in Warsaw. Then German soldiers take him away to be measured and assessed. They decide that Peter is racially valuable. He is Volksdeutscher: of German blood. With his blond hair, blue eyes, and acceptably proportioned head, he looks just like the boy on the Hitler-Jugend poster. Someone important will want to adopt Peter. They do. Professor Kaltenbach is very pleased to welcome such a fine Aryan specimen to his household. People will be envious. But Peter is not quite the specimen they think. He is forming his own ideas about what he is seeing, what he is told. Peter doesn't want to be a Nazi, and so he is going to take a very dangerous risk. The most dangerous risk he could possibly choose to take in Berlin in 1942.