It sounds like he could be in some sort of pain. It could be gas, but since it’s been a few days I would prob take him to the vet. Is he showing any selectivity with what he’s eating?
Can you take his temp? Do his ears feel cold or warm? Cool is OK, but if they are cold it could mean his temp is low. You can also do a rough check to see if he’s gotten dehydrated by grabbing a handful of his scruff and seeing if it snaps back into place or kind of slowly rolls back. The later means he is dehydrated.
Some bunnies do go through more quiet days (for example some buns get very cranky when molting, or when they get bored with their environment), but since it’s been going on multiple days and onset suddenly I think it’s worth checking for a medical cause.
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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.