RE: the vet appt, make sure both rabbits go to the vet, as taking only one can break a bond since the departing rabbit comes back smelling very different.
There are a number of possibilities for the behavior, but in general, just keep in mind we are humans and they are rabbits, so how we perceive the world and relationships is very different from them– ultimately, what may seem “strange” to us can be totally normal for them, so I wouldn’t stress too much about small interactions and try to read into it. They are not humans.
RE: explanations for some of the tenseness in the bond, it’s very possible the bond itself is not very stable. It is relatively new and it sounds like they were not the most compatible match to begin with, so this may be a natural conflict coming from their hierarchy and whether it works itself out or not smoothly– only time will tell. It can also be that since he may be ill, this could just be making him act differently, therefore there’s some increased tension between them.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.