Why can’t it be his first taste of combat action? When was it established that he was in some sort of elite and experienced stormtrooper unit and only the best of the best ever went into battle with Ren? If stormtroopers are commonly used as cannon fodder wouldn’t they constantly be bringing in some new reinforcements anyways? I don’t think Finn’s behavior really requires any more of an explanation other than what they show and what he tells us in the film.
Who bothers to memorize the serial number of cannon fodder?
It appeared that Kylo sensed something wrong with him during and after the battle? He maybe did some quick intel on that trooper upon their return?