I've done some basic looking around and found, as you did, that numbers on this are not easy to come by. I suspect you will never find an accurate version of the numbers you want for two reasons:
German-Americans often did not advertise their Germanness during the world wars, for reasons that are easy to understand. Thus any official records probably extensively underestimate the number of German-heritage/German-born American soldiers simply because they didn't admit to that when they signed up, up to and including lying about their place of birth. This is anecdotal, but my great grandfather was born in Germany and fought for the US in WWI. During that period he refused to speak German or teach it to his children, and outside of keeping a German surname, the family effectively ceased being German in any cultural sense. I suspect that kind of experience is common. As for how they would get away with lying about that:
The American military had shrunk enormously between WWI and WWII, and was faced with the prospect of recruiting hundreds of thousands of men to fill the ranks. The German-American population is huge and the War Department had good reasons for ignoring ancestry if it meant gaining tens of thousands of recruits.
After the war much of that population considered themselves American more than anything else, so there was never much of a push for recognition. For that reason and the ones I listed above, the numbers you're talking about either never existed, or were highly inaccurate. I do think it's unfortunate, because there are few examples of the American melting pot more poignant than someone fighting against their former countrymen, but in this case there are also very logical reasons as to why it's so hard to find information on them. I hope this helps, and tell your father thank you for his service.