Welcome and thank you for adopting this bunny!
Buns can definitely be a bit picky about pellets, especially if they are used to eating a brand that is more calorie-dense.
Here’s some info that will help you read the labels and decide: https://wabbitwiki.com/wiki/Pellets
The main thing is to look for a pellet that has at least 18-20% fiber, so usually the first ingredient should be a grass hay (timothy is common). Also make sure it is not the museli type that has seeds and other things in it, as this encourages selective eating.
Does the bun also eat hay? Grass hay is the most important part of the diet, so as long as he’s eating plenty of that he will be OK skipping pellets for the time being.
With a new food, it’s important to introduce it slowly in small amounts, and be patient if the bun doesn’t want to try it right away. Sometimes they need to be offered it a few times before trying it. I had a bun that wouldn’t eat her joint support treat at first, but once I broke it up into smaller pieces she tried it and then learned to love them.
Also if you are new to rabbits in general, there is lots of good info on diet, housing, health, and behavior here to get you started: https://binkybunny.com/house-rabbit-information/
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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.