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Cas believes him.
Sam clings to that. He's not crazy, Dean is getting worse, a little bit every day.
On the outside, he looks like he's doing really well with his zen bullshit. When he's awake.
But he has nightmares every night now. Every night he calls Sam's name, desperate and painful, and Sam can't do anything but listen to it echo down the empty bunker hallways with his heart in his throat and pray that if he can't cure his brother, Dean takes him down first, so at least he doesn't have to see the rest of it.
It's not just the nightmares, either. It's the thrill of the hunt, the way Dean schools his face after a kill if Sam is in the room, but grins like a maniac if he doesn't know Sam is watching. It's the little kindnesses that Dean gives him because that's what he does when he's preparing to die: letting him drive the Impala so he can get used to Baby's quirks again; giving Sam one of his photos of mom; saying thank you all the fucking time.
It's killing Sam.
But at least Cas believes him. That is a small comfort.
He can tell Charlie doesn't – she's buying into Dean's well-balanced, on-the-right track bullshit. She thinks he's okay because he can recognize unleashing the Book of the Damned to save him is going to cause bad things to happen. Really bad things.
But you know what? Sam can deal with bad things. As long as he has his brother, they can deal with any amount of really bad things. It's a risk he is willing to take. He'll clean up the mess, after – if he's still alive – but he needs his brother, and every day that goes by with no cure is wearing Dean down, slowly but surely. Sam can't wait around for some other miracle cure. This will work. He has a plan. Dean will be saved.