"You?" If he sounds shocked, it's because he is. He can't even process the idea of it, a world where Stiles doesn't care about Lydia? No way. "He loves you...he's always loved you." And maybe it was stupid and naïve of him to think that was all they needed, to hope that it was enough, but-- it's just who Scott is. It's a core part of what makes him, him.
"...bringing Allison back."
Those words hit him hard enough to almost leave him breathless. Because...it's eerily like what he wonders might have happened to Allison, why she remembers everything all wrong, but still knows certain things, at her core, that she can't deny. He knew there was magic in the world, that it comes with a price. What if...
"What?" his voice is hollow, empty, and barely above a hushed whisper, etched in disbelief. "Why-- why wouldn't he-- Lydia, what--" Panic creeps back to settle in his chest. He's torn on how to even feel about this. Having Allison back? Yeah, it's the wound that never healed, it's one thing he'd do almost anything for. But not at the risk-- the price of losing Stiles in the process. That's not what he wants. That's not what anyone ever wanted.
He's scared and angry, hurt and hopeless, he's too much at once and he doesn't know what else to do except hug Lydia close and rest his cheek against the top of her head. Tears roll heavily across the bridge of his nose and into her hair, but his sobs are silent, the only evidence being the way he shake from the force of it. He can't do this. He can't handle any more loss. He just can't.