In the moment, it hasn't even occurred to him that she knew his name when he hadn't known hers. If he thought about it, he might start to ask questions, but it doesn't even come to mind because he's lost in the moment entirely.
"Cassie," he repeats breathlessly, nodding, "Cassie, okay, Cassie." And then he's kissing her again, his hands in her hair as he backs her up against his dresser to set her down on it. He doesn't need her last name, not really. Not unless they're going to fuck and he's not quite there yet, so that can wait for now.