Good question. First he never met his great grandfather in life so didn't even know what he looked like. His Dad showed him a picture of him with gray hair and glasses and he said “No, he didn't look old like that and he had no glasses”. His Dad searched for a much younger picture, when he was in his twenties or thirties, and the boy responded, something like, “yea that's him”. He also was never told about how his Dad called his Grandfather “Pop”. The man in Heaven who introduced himself told him his Dad called him that.
The powers of darkness can easily do this same thing.