Susan, this is for you!
Dean watches Sam steady little Blaine on his knees as the baby bounces around on short legs, making blissfully happy noises. He whirls on Castiel, who stares back at him with an unreadable expression. That just makes him more pissed off, and he finds himself yelling while Castiel’s voice is low with barely suppressed anger.
"What the hell, Cas?!" he demands.
Castiel’s brows furrow. “Dean, do not use such vulgar language in front of our child-…”
"Exactly! When were you going to tell me we had a kid together?”
"I did not have the opportunity before. That’s why now-…"
"He doesn’t even look a thing like either of us!" Honestly, curly hair? "I bet he’s not even mine!"
The moment those words leave his lips, Castiel’s expression darkens. There’s a loud rumble overhead, like thunder rolling, despite the sky outside the motel window being blue and clear. Dean feels his heart speed up with fear, but thankfully Sam intervenes before things get out of hand. “Hey, hey,” he starts and pauses, apparently not sure how to continue. But thankfully Castiel turns away from Dean to look at him just as Blaine whimpers and squirms.
The rumbling immediately stops and Castiel moves forward to pluck Blaine from Sam’s grasp. Without even looking at Dean, he sits down at the edge of the bed and pulls a baby bottle out of his pocket. Dean is pretty sure it wasn’t there before, but decides not to question it as Castiel cradles Blaine close and feeds him. Blaine greedily grabs at the bottle with his tiny hands and swallows the milk it provides in big gulps.
Dean slowly sits down on the other bed, watching them and trying to ignore the constricting feeling in his chest.
"Blaine is your son, Dean,” Castiel says quietly. So quietly he almost has to strain his ears to catch it.
Dean runs his fingers through his hair and sighs. Slowly, he looks up at Castiel, staring into his deep blue eyes. “Start from the beginning, Cas.”
Unfortunately, Castiel doesn’t get the chance to explain where Blaine came from.
As soon as he finishes his milk, Castiel holds Blaine against his chest and pats his back until he burps loudly. Almost immediately, his face scrunches up and Castiel announces that they need to pick up supplies for Blaine. Which is how Dean finds himself pushing a shopping cart, Blaine strapped in the baby carrier, Castiel grabbing baby formula and diapers and other things off the shelves Dean is trying not to think about. Sam trails after them, suppressing a smile and being entirely unhelpful.
"I don’t get it," Dean mutters, watching Castiel dump boxes and packages into the cart. "You have the kid’s bottle, but not all the other stuff?"
Castiel stares at him for a long, unnerving moment and Dean tells himself it’s not relief he’s feeling when he finally looks away. “I didn’t have time to bring much with us.”
"Why?" Sam asks. He leans over Dean’s shoulder and makes faces at Blaine, who laughs even despite the state of his soiled bottom.
Castiel’s expression hardens. “Zachariah,” he says.
It’s all he has to say.
"I hate that asshole," Dean growls. He looks down at Blaine and his small, round face, feeling a wave of protectiveness. The baby beams at him and he realizes just how screwed he is, whether Blaine really is his child or not.
When he looks up, Castiel is gazing at them with a fondness that scares Dean more than he’s willing to admit.
"Let’s hurry up and buy this junk before the kid starts crying," he says quickly and pushes the cart toward the check out line.
The title comes from “Cough Syrup”, originally by Young the Giant, later covered by Blaine Anderson (Darren Criss) from Glee.