"Oh, um --"

Trapped between his instinct to be polite -- less to conform to arbitrary social conventions than out of a desire to be nice to people -- and the knowledge that he is really shitty company right now, he shifts from foot to foot.

Politeness wins out, and anyway he should. Probably not be alone right now. Thoughts keep bothering him, and while he can tell they're not other people's, he's not sure if they're really his. And he can't make them go away. "Well, sure."