with my current (second) character I also have him as a rival and was wondering, if he actually had a "questioning believes" quest after "bitter pill" (I thought this was the case while befriending him) ... maybe these issues are related. your quest doesn't work and in my game it doesn't even show up. sadly I don't know how to fix this, but I would guess this is related to the rival status.
For me it just stayed at the Questioning Beliefs quests telling me to talk to Fenris, and did so until the game ended; he never did respond, and I tried using the debug to initiate the dialogue, but didn't work out.