not sure if NA papers show parents but would think the death cert should.
if you know the city , google vital records for the city in Wisconsin where he died and it will say where to write .Before paying ask if the record shows parents.
DOD 6/8/1940 If you can get church records all the better.
otherwise, if you cannot get any info from living relatives, you will have to contact Germany.For reliability use govt or libraries there. The birth cert will have parents name /also marriage cert. Google vital records for the town in Germany for contact info.