Hi there,
I agree that the vet can help you assess. My vet actually prefers buns on the thinner side… you should be able to feel the spine and hips, just not super sharply.
I think the key is also to determine whether he is losing weight for an unexplained reason, or whether he is just a trim bunny. Weighing regularly at home can help you assess this.
Since he is so sensitive to pellets, he might have some underlying GI issues that might make it harder for him to gain weight. He is quite young still, so it might be that he was still adjusting to an adult diet when he was having issues before. Potentially increasing his hay variety to include richer hays might also help (like oat).
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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.