He laughs a little, and then—
The most immediately obvious thing about Sean's mind is how embodied it is. His experience of the world is very heavily sensory and almost completely nonverbal. He looks at Leah and sees—he's still reading her mind too, but even if he weren't, he has an intuitive grasp of body language that feels like just directly perceiving the way someone feels in their posture and expression, and that perception is part of his sensory experience on almost the same level as colour or texture.
On top of that, then, he's also thinking about other things. Like what he saw when he first let her out into her own head. He's still a little in shock over it. There isn't much of her but all of her that there is is so... he's not thinking the thought in words, and probably couldn't come up with a word for it if he tried. But when he thinks about her, about who she is and what she's like and what it's like in her head, there's an edge of giddy disbelief to him, like he can't figure out how to reconcile his previous understanding of the world with the knowledge that it contains someone this amazing. The concept that the most wonderful thing in the world is people, the definition of sonder, those things resonate with him on a deep level—and even the rest of her, the drive to understand the world that he himself does not feel, he still sees as precious and good and important.
There are other fragments of thought floating around in there too. Memories of bits and pieces of this conversation from his perspective. The sensation of what it feels like to speak out loud in words when they're not remotely your native format. The firm conviction that of course he's not going to fuck with her if he takes ownership of her for this purpose, that it would be—something like unthinkable, to do that and then treat her as someone he happens to have power over instead of as someone he is knowingly and willingly responsible for. This sense of responsibility is novel and largely unexamined but very, very, very deeply felt.
All together, it's kind of a lot.