My parents and doctors say he is not especially long for this world. My grandfather is the quiet, reserved type. (Not uncommon for a German) I have a few burning questions, but I feel it would be awkward asking delving questions out of the blue, because I have never brought up his war history in conversation before.
What are some starter questions I can open with without making him feel too uncomfortable, and what would you guys like to know?
Any and all suggestions would be greatly appreciated. I've always had a dignified respect for him without ever questioning why.
(I tried posting in askreddit but it got removed for being too specific, Can you guys help me out?)
Be sure to gather up any relevant information you can about where he lived, what regiment he was in, where he might have served, and what the experience would have been like. That'll help you prepare questions and trigger his memories if he decides to open up. Pictures are a wonderful memory jogger. Remember, he's an old man and things are going to be hazy regardless. Try to ask what historians call qualitative questions instead of quantitative ones. So, avoid asking for a timeline or where he was on January 23. That's difficult enough at 19, let alone 90. :)
Is there a friend or family member he is/was especially close to in Germany? "Oh, Hans! He was a real troublemaker..." will lead to questions about how they stayed in contact during the war. Or tell him you're interested in German-American relations and let him bring it up. Something to break the ice. The accent is the kind of thing to ask about once you get to how German or American he considered himself and why he joined the military. How differently was he treated? Did being German ever serve as an advantage? Did they need translators or cultural ambassadors? All that makes for a better conversation than asking about the German American Bund (Nazi sympathizers), though if you think he'd be comfortable talking about that, it could be fascinating.
The main thing to stress is how much it means to you to learn about this stuff. It's your family history and all. Most people love that the young folks are interested as long as it's nothing traumatic. He'll fill in the blanks that you want to get the information before he's gone.
I'd start with little things, such as his rank and where he served. If he seems uncomfortable, stop there, but, if not, start asking for personal stories. Just out of curiosity, what did you want to ask him?
Be sure to ask him what little stories or tibits there are that HE would like to share as well. Maybe little details that have somehow stuck with him through the years.
I just wanted to add on to this, as well as all the great advice already given. Should he appear upset or frustrated, don't take any of these emotions personally. So long as you're handling the situation calmly and in a polite but interested manner, he's not going to get mad at you for asking, just upset with his loss of memory over time.
Also, if he's anything like my German relatives, just don't bring up very specific questions about say the nazi sympathizers over here, or what he thought about the holocaust etc. My cousin made him very upset by asking those types of controversial questions. Keep it simple, and let him do the talking :)
Ask him whether he knew about the internment, it might have been a deciding factor in why he joined the US Army.
http://en.wikipedia.org/wiki/Internment_of_German_Americans
I also recommend that you try to record what he tells you, audio would be a good idea. I regret not using a tape recorder on my elderly relatives when I still had the chance.