Why does Winston feel that the man sitting next to him in the cafeteria is “not a real human being but some kind of dummy”?

The man in the cafeteria at Winston's workplace, who is about 30, is sitting in such a way that the light hits the lenses of his glasses so that his eyes are obscured. He looks as if he has "blank discs" behind the glasses. However, the most dehumanized aspect of him is his speech: he goes on and on, endlessly pouring forth language, but Winston can't distinguish most of the words:

Just once Winston caught a phrase—"complete and final elimination of Goldsteinism"—jerked out very rapidly and, as it seemed, all in one piece, like a line of type cast solid.

The man is frightening because he talks and talks and but says nothing of substance. He is merely stringing together phrases of propaganda that have been brainwashed into him by the state. He is saying words without any real meaning, and he certainly isn't thinking or evaluating what he is saying.
One of the things that I love about 1984 is that this man, who sounds like a robot ("dummy" today might be an offensive word to the deaf, though Orwell doesn't mean it that way) is a real type: Orwell nailed it. Every time I read this passage, I think of a man I sat next to on plane in 2006: he was loudly insisting to the man beside him that there would be no real estate crash (just before the great recession, which was caused by the housing bubble). There was just no penetrating his mind with critical thought. He was like Orwell's robotic man, simply spouting propaganda that didn't make any sense and wasn't connected to rational thought processes, and it seems to me that while our situation isn't Orwell's dystopia, many aspects of the book can be related back to our own lives in ways that are worth contemplating.

In chapter 5 of 1984, Winston is sitting in a cafe and overhears the conversation of two men. He can only catch a little of what they're saying, but it's clear that the men are loyal members of the Party, unthinkingly spewing out the official propaganda line. When one of the men calls for the complete elimination of Goldsteinism, Winston ruefully observes that he's little more than a dummy. By this, he means that the man has lost all capacity for independent thought; he's not really a human being in the fullest sense of the term, with the ability to think for himself. He's nothing more than a creation of the regime, a brainwashed automaton blindly doing whatever he's told and believing every lie told to him by the Party.