Dean knelt down and held on tightly to Sam while he watched the angels falling. Sam, being very distressed and in pain, sat with his back pressed against the Impala and was hardly able to keep his eyes open. Dean had his arms wrapped around Sam protectively and had his body shielding him as if he was afraid that an angel might fall right on top of Sam. Sam jumped every time he heard a crash. Dean didn't know what to do. For a long moment, he forgot how to move. He shut his eyes tightly and tried to concentrate on his prayer.
Cas, you son of a bitch. What have you done this time? You…you okay? Cas, if you can hear me, please… just come here now.
It was difficult to focus with residents of heaven being evacuated over their heads. And then he mentally cursed himself because obviously, if Castiel is falling, he's not going to hear Dean's prayers any more. And this led to a full minute of internal panic attack because how is he supposed to reach out to Castiel now? When he finally could form coherent thoughts, he realised that at this moment, he could be losing the only two living souls that he cared about. He had to set his priorities straight. He can't possibly help Cas right now.
Sam. You gotta watch out for Sammy. He comes first.
Without uttering a word, Dean tugged on Sam's hand as Sam groaned in pain and stood up shakily.
"Dean… where's Cas?" he asked when he realised that this meant that Castiel must have fallen too and could have crashed anywhere.
"I don't know."
He couldn't be bothered about the way his voice sounded wrecked because that was exactly how he felt. He put Sam's arm around his shoulder to steady him and almost fell forward when Sam bent his body and screamed in agony.
"Hang on, Sammy. You're gonna be fine. I'll fix you up and you'll be okay. Just… hang on."
By this point, tears were streaming down his face as a dagger drove through his heart at the sheer helplessness he felt. Dean really didn't know how to save his brother. He didn't know how long Sam would last. But he mounted Sam into the car and drove away anyway although he didn't know where exactly he was supposed to go.
Dean hesitantly drove to the Man of Letters bunker that night when Sam insisted that no medical treatment can do him any good. He then assisted Sam to get into his bed and perched in a chair next to him all night, watching Sam drift off into a fitful sleep. Sam woke up countless times in the night, his hands gripping his chest like he suffered from a cardiac arrest. Dean did all he could. He made tomato soup and grilled cheese sandwich that he knew Sam likes. But Sam could barely sit up, let alone eat. He held a bucket out for Sam whenever he puked his guts out. The only reassurance Dean got was that he wasn't puking blood any more. And when Sam trashed in his sleep like he had nightmares, Dean just held his hand and reminded Sam that he was right there. That everything will be okay. He didn't believe it himself but if it's the last thing Sam was going to hear, he needed to make sure that Sam didn't feel scared and alone. Sam had his brother next to him at every passing hour that night as though Dean tried to prove with his actions what he -more often than not- couldn't express in words. That no matter which way this ends, no matter what Sam believes, Dean will always be there and care for his little brother. No one and nothing can change that.