Okay, I'm retrying this right now and I guess I have never before tried to talk to him during his first event on my Bjorn playthroughs *facepalm*. This seems to be the trigger that allows you to chase after him after supper. It seemed so pointless after the first time I did it - he runs away after like two lines anyway, how was I supposed to know it changes anything?
Sorry I was suspicious of you.
I really need to spend some time and figure out how to properly butcher renpy games to check for routes without having to go through 358 possible combinations of decisions.
I'll play through the event and get back to you.