I appreciate the compliment, though I'm not sure your faith is justified.
At one level, I would guess that the Nuremberg trials were well documented. On the other, I could see how they might want to keep secret the names of some involved, for their own safety.
Maybe aalan94 will chime in, or you can contact him, he's not a WWII guy, but he knows how to research.
One thing that makes me think the stories could be true. If he was a native German speaker, it would be smart to have him involved with the security. Either as a translator, or simply to eavesdrop on the prisoners.
If you don't have his service records from WWII, you can get them from the government. It might take a long time, but worth a shot. Of course, lots of records were lost in the 1974 fire, but maybe you will get lucky.
https://www.archives.gov/veterans/military-service-recordsIn the meantime, talk to as many of your living relatives as possible. Get their stories, even if you think they may not be accurate. Between family legends and official records, maybe the truth can be found.