I would just list him as "Pinto" since you don't have any proof of what his breeds are. He is beautiful, btw!!!! Conceivably you could actually get him reg'd through the pinto horse association as well...since you don't have to have lineage, just certain traits, which he certainly fulfills. Good luck selling him
Firstly, Princeton is a beautiful boy and has very good movement in the roundpen. With his being decidedly uphill with those high withers I doubt he would have any Quarter Horse blood unless he has some age on him. Your description of his hooves being big and flat is indiction of maybe some Thoroughbred or more possibly some Draft breed blood.