I have a couple of examples. I have the literary magazine from high school he worked on and contributed art, which he signed, to. I also have the high school 5-song CD, signed by david and Bobby Ker.. I have no photographic evidence, but I may one day take a picture of the CD. Oh, and the staff page of the magazine, just to prove I worked on it with him. But I'm at work, and I don't have a camera handy (nor the CD or magazines...but they are at home). Done rambling.
EDIT: Well, damn it, someone's high school demo CD is better than mine! Was browsing eBay and found this auction, which includes the high school CD and three others, and photos of them all.
http://cgi.ebay.com/DAVID-COOK-CD-COLLECTI...bayphotohosting