As unusual as it may seem to her- and he's fairly confident that that is what is prompting her question- he does actually know how to smile. It's something he rarely chooses to exhibit.

"Yes, I am." He has much more to do, with who he needs to find and apologize to, for what he and Kal-El did, but it no longer seems as difficult or unlikely to accomplish as it did at the start. He now has something more than an empty hope and absolute reasoning to go off- evidence.