Well he "joined" the Hitler Youth at age 14 in 1941. Wikipedia says from '39 on there was a law so everyone had to.
Other than that, no affiliation with the nazis (except for being German) can be tracked. Now I know many people who deeply were involved later on denied it, but you can't just say he was a Nazi. Except everyone who lived in Germany at that time is a Nazi to you. But that would be a different discussion