If his parents were Latvian citizens (Russian before 1918) they still probably considered themselves part of the German cultural world. Over more than 1000 years of Jews living in Germany there was significant 'mixing' of German and Jewish blood. My understanding is that the Nazi policy was that anyone 1/4 Jewish or more was considered Jewish, but there was some opportunity for well connected people who were only part Jewish to get exempted from this.
10 arrested at Israeli Arab protest in Ramle (Haaretz)
from the article: U.S. finds long-lost diary of top Nazi leader, Hitler aide