Hmm, if he’s sitting in the same spot all day, he is most likely in pain. Shredding cardboard can also be a sign of pain. I don’t think the cardboard would have upset him.
Soft poops can sometimes go along with gas, which could be causing him pain. Some bunnies are also very sensitive to stress, so it’s possible the stress of the vet visit set off his tummy a bit. Is he eating his hay? What about his veggies? It could be that his teeth are bothering him, so he’s not eating enough hay.
I think a vet visit (or at least a phone call) is in order to determine the cause. How long have you had him? If nothing has changed recently with his diet (aside from removing treats and cutting back pellets), then I don’t think it would be a dietary cause.
If his appetite drops or he stops pooping, then he will need to go back to the vet right away.
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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.