Contact these two Texas A&M professors mention in the article:http://j.b5z.net/i/u/2084131/i/Universi ... n_POWs.pdf
They should have knowledge of the available resources.
You may find something in the US Army Provost Marshall records at the National Archives (NARA), College Park, Maryland such as camp rosters. You'll need to visit there in order to go through the file boxes.
NARA also has US military keep interrogration reports on thousands of German POWs. Years ago, I remember seeing a binder with the names of the Germans for whom there was an interrogation report. If there is an interrogation report on your grandfather, it would probably list the interrogation center (and perhaps interrogator) that conducted the interrogation.
Once you know what camp(s) your grandfather was in, there is a long shot is there a mention of your grandfather in the Army Counter-Intelligence Corps files, too. If I have seen some of these reports that mention the activities of different Germans by name - blacks/Nazi, grays/neutral, and whites/anti-Nazi.
You can also write the International Committee of the Red Cross (ICRC) in Switzerland. During the war, they kept index cards on all POWs reported to the ICRC. They should have an index card on your grandfather. The card might mention a camp.
Did your grandfather mention any kind of outside-the-wire work details he was on? If he worked on a somewhat unique POW detail that could help you locate the camp.
I once was told that I was vain, but I knew that vanity was a fault, so I gave it up because I have no faults.