Asked a simple question about when his symptoms began, a man gives a long, detailed, meandering account full of unnecessary tangents, and each time you try to redirect him he briefly returns to the point before digressing again. Eventually, several minutes later, he does answer the original question. What is this?

This is circumstantiality: the eventual answer does arrive, but only after a long, detailed, tangential account with excessive unnecessary detail at every step, the overall goal of the conversation retained throughout, and attempts to redirect only briefly successful before the digression resumes — exactly as described here. It is not flight of ideas, in which the connections between ideas are driven by chance and the overall goal of the thought is lost entirely, rather than merely delayed. It is not tangentiality, in which the point is never reached at all — here he does eventually answer. And it is not perseveration, which involves a specific earlier answer intruding into a new, unrelated question, not a long-winded but eventually goal-directed account.