I think a DNA test is your best bet. It wouldn't surprise me if there is something besides pit bull in his blood lines. Some of the pit bull crowd have more recently started gravitating towards mastiff breeds such as Cane Corso, Dogo Argentino, and Presa Canario. Occasionally a cross between a pit bull and one of these newly trendy breeds will turn up in rescue. Really, though, your best bet is a breed test.