Author's note: Hey this is my take on how Dean would respond later on, I cried during with that over coat scene, it was a very tasteful way of saying good bye to a beloved member of the team. This one is kinda a downer but anyway please enjoy :)
Dean looked at the soaked fabric in his hands, this was it. This is how it ends for Cas, he's died for Dean on more than one occasion, but he has always come back. Dean ignores that tug in his chest that tells him that was the last time he would see his Cas, at least as Jimmy. Laying his left hand on top of the coat, pressing that hand firmly as he would have done to Cas had he been there. As Cas had done to him.
"You damn child." Turning away from the reservoir, he didn't even glance up at Sam and Bobby who just seemed resigned to what the hell had just happened. First in the car Dean couldn't bare to let go of what was left. This symbol of the angel. Reaching his hand up he placed it over the scar on his shoulder. He could still feel the heat that lingers with burns, the nerve endings remembering the heat that caused the tissue to swell. He slide himself into the drivers seat, taking a moment Dean breathed deeply through his nose "Cas, what the hell man." He reached up to the wheel as Sam entered the impala face full of empathy, Dean could feel a speech coming on.
"Look Dean I-"
"Save it Sammy, I'm fine. Just leave it be okay?" Knuckles growing white. Sam put his hands up defensively.
"Okay, okay." Deflating Sam slouched down into his seat as Bobby walked up to the driver's side, he looked at both of the boys nodding grimly. Dean didn't need to be told where to go, there was a nonverbal understanding as Bobby patted the roof of the impala as he moved to walk to his own car.
The day continued in an almost eerily casual manor, there were a couple times where they discussed what needed to be done, what to look for in the papers and what to research. Their only lead was Leviathan which was a pretty good start compared to some of the cases they've dealt with over the years. Later when Sam had decided to call it a night and hit the hay Dean followed suit, leaving Bobby to himself.
Dean lay on the couch in the living room, trying to find a comfortable position and failing miserably, his shoulder hot and sensitive with each shift. After what felt like must have been hours he finally managed to doze off into a light sleep but found himself dreaming. It was of that same pond and dock Cas had visited him in what must have been two years by now. The interaction played itself out in his mind now truly as a dream. The scene no longer relaxing and peaceful but instead as the realization struck him he woke with break neck speed. The hunter opened his eyes to stare at a devil's trap painted on the ceiling in glow in the dark paint, the sight was reassuring that he was in fact awake. As he came too the realization that he had sweated through his shirt was not a welcome one. Sweat mean't new shirt, new shirt mean't getting up. Damn.
Placing his bare feet on the carpeted floor he lifted himself up from the now moist couch, the very same where he had lay broken and healing too many times to count. It was also the same couch where Dean and Castiel had shared their only kiss. It hadn't been much as it was a brief lapse of judgement, as he had called it after they separated. In retrospect Dean wished he hadn't been so pig headed and down right stupid, because now he was left with feelings that he didn't really want to think about right now, but ones that would never really go away. Reaching his duffle bag not far from his makeshift bed he began shifting through it when he caught site of the angel's over coat he had placed next to his bag.
It had dried now, for the most part some moisture still clinging to it's material as Dean noticed with his hands. The worn fabric pliable and well used. He remembers grabbing the lapels and tugging Cas close to him. His throat grew tight, shoulder aching as he just stared at it, he could feel tears but he fought them, he fought them like everything else. He wanted to change so much in this instant, every wrong move played itself out, it almost took his breath away. His grip tightened and he brought it up to his chest as he pressed it there. "Damn you, Cas." He knew deep down in the very bottom of his soul, he had not seen the last of Cas. They would fix things as Cas had promised and forgiveness, Dean owed him that much.