He has lost the ability to open his mouth and speak.
At first, he simply kept silent. Not interrupting, not cutting in, listening patiently while others spoke -- that was a kind of courtesy. He smiled, nodded in acknowledgement. When others asked him something, he could still open his mouth with composure and answer cheerfully.
Later, he grew accustomed to the feel of silence. Others liked to talk, so let them express themselves. There would always be people willing to put themselves on display, so leave the stage to them. As one fervent and intense play after another unfolded, it wasn't so bad to be a quiet onlooker.
Later still, he came to feel that speaking was something that consumed a great deal of mental energy. The world was clamorous, people's hearts unsettled and noisy; he only wanted to stand placidly off to one side, like a statue, without saying a word. If you don't speak you cannot say anything wrong, nor do you have to weigh your words before opening your mouth. Even when he had no choice but to speak, he would wrap things up in two or three short sentences and bring the topic to a close. If that didn't resolve the matter, he would package his thoughts in extraordinarily long-winded sentences, with no regard for whether others could understand him at all.
In the end, he lost the ability to open his mouth and speak. He wanted to say something, but no longer knew how to express what was on his mind. He became the most inconspicuous person of all, having lost any sense of presence whatsoever.