Stopping where he stood when he heard the kid speak again, Stephen turned around to look at him. This was obviously not working. It was becoming clearer and clearer that Stephen was well within his rights to make a snap judgement about the kid's sanity. He was more than likely mad, and if he wasn't mad he was desperate. Probably a bit of both, if Stephen were to put money on it.
He made an overstated, slightly camp gesture of exasperation that involved putting his hand on his hip and sighing rather dramatically, albeit silently. Frankly, he wasn't even sure if the kid was actually registering this, since he'd totally ignored Stephen's previous communication. There might not have really been a point to him being so theatrical now, but damn if he wasn't going to at least try and show it.
Half of him wanted to Disappate right there. The other half knew that a very loud bang in the middle of a busy street would probably cause some kind of trouble, maybe even for the kid, considering he was the one standing in nearest to it whilst talking to himself (from a Muggle perspective). Stephen planned out the most expedient course of action and walked up close to the boy. "Yes you did. Now quit making a scene of yourself." He then started again to walk away.