Johnny says absolutely nothing. He stares, tight-lipped, at the Doctor, listening to the sudden outburst without an ounce of comprehension. If this is playacting, it's obscenely elaborate. If it's not, then... he doesn't know. He can't afford to let the possibility seep into his mind. He can't afford the luxury of hope.

A lot of this smacks of things Charley told him, but he can't quite remember it all clearly, and what's more, it's entirely possible that Zagreus has been in his mind since then, found that out, is now using it to some sick advantage. So he's silent.