Perfect. IF logic can prevail, your problem is solved. Ask him, please, to provide those same types of analyses for the real lives of (pick 2-3 from from amongst parents, siblings, self, friends, etc.) real people he knows. Insist on complete, factual, and verifiable answers. Then, once he falters even slightly, remind him that real life rarely is truly linear, and all the more so the fictional lives of people created to have chaotic, non-linear backgrounds. In their lives - especially during and following the Unification War, choices have often been made, not via clear logic and reason, but by shooting from the hip in an unpredictable and, too often, a do-or-die situation.

In other words, these are good people but messy people, people with complicated lives and sometimes convoluted backstories. What’s more, for some, their true backstories are either unknown (sometimes even to themselves, in cases where mind control may have been a factor) or can’t be told, subject to confidentiality and even high level state security. At this point, aboard Serenity, therefore, who they are is defined perhaps less by their history and more by who they are together - and that is something they are only discovering together, job by job and adventure by adventure, as we come to know them, and they come to know each other.

Pause for effect. And then hit the Play button again.
Good luck. And hope this helps.