As much trouble as Will has with social interaction, he has learned to be quite resilient. Though in this case, perhaps the reason he has stayed in the conversation is more a mixture of curiosity about Cambridge added to the fact that he's used to being the one people make their excuses to and leave behind, rather than the other way around.

"Really?" He frowns. "I was under the impression they were more serious than that. And if it's changing the future that you're worried about, isn't any change essentially the cause of a sort of a paradox? Two parallel but, ah, conflicting realities, I mean. Not so much a case of the present conflicting with the future but of your past creating an altered version of itself that by its nature affects the future because it is your past. If...that makes sense."

He may not know time travel like the Doctor does, but he's at least read up on the subject--which in Will's case means he's memorised a ridiculous amount of information about the theories behind it.