As long as he's eating a bit his digestive system won't shut down but after an op most rabbit should be back to eating normally the next day. You've said how much he's eating now but how does that compare to what he was eating before? Have you asked the vet about his eating pattern now? How much is a little hay? The pellets aren't too important as part of their diet its more of a suppliment, the majority of his diet should be hay, grass, veg with just a few pellets. Is it just carrot he's having as a veg? Sorry for all the questions but it may help answer your problem.

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