Unless you do DNA testing, it's impossible to tell. You can only go by looks, and he looks like a pure rottie to me -- but you never know, might be something else back there! My friend had a dog that absolutely looked like a 100% purebred rottie -- but, in fact he was 1/4 American staffordshire terrier. Never would have known it if they didn't tell me that their other dog, a solid black dog that looked nothing like a rottie, was his mother. Nothing wrong with calling a purebred-looking dog (with no proof that it's purebred) a purebred. If it looks purebred, might as well be called purebred.