It didn't take an expert to know that the other teen, masked or no, was upset. It was evident in his stance, frown, and guilt laced voice. Billy, at this point, had calmed down enough to start to realize that he'd just treated Kid Flash the way others had treated him after his incident.
And really... outside the shock and initial confusion, it was pretty cool. He was in St. Louis. He'd never expected, when waking up this morning, to do such a thing. Sure, he could theoretically do it with his own powers... but he wouldn't, and that was the difference between the two. This kid, whoever he was, wasn't scared to be who he was... was actually proud of it. And Billy, well he was secretly ashamed, even though he did kind of like some of the perks that came with the package.
"Hey," he said softly, stepping forward and risking a hand on the other teen's shoulder. "I'm sorry. That was rude of me. I-I guess I thought you were just pulling my leg with the costume and all that. I didn't expect this. It's cool though... really."