There's nothing he can say to prove he's not who they think he is. He's tried politely, with increasing panic through each interrogation. There are too many coincidences, too many correlations between his truth and their myth for rational explanations.
But that doesn't make it true… right?
They want him to be their John, need him to be with such frantic faith that for one brief moment he's wishing with them.
He leaves that out of the mission reports. And therapy sessions. And fireside chats with friends who won't-- can't understand.
Because he's not sure he isn't their John anymore...