How did you guys react to him about Duck being bit? Did you constantly bring it up and kept telling him he has to deal with it? It's true but it hurts him to hear it from you over and over. Fighting him on the train will not help one bit haha. And if you choose Katjaa or Kenny to shoot Duck, Katjaa will go out there alone and as she's walking away you can put your hand on Kenny's shoulder and it says at the top "Kenny won't forget your compassion" or something like that.
I don't think it's all just the stat screen choices that matter, those are the big players but a lot of the little dialogue and and the way you handle yourself also plays a role.