Sorry it took a while for someone to reply to this, I thought I had already but it must not have gone through!
Some rabbits do take a long time to warm up, and they are all different personalities so there will be variability in how much interaction they want from you.
Since he will take a treat from you, perhaps you can do even more hand feeding? You can hand-feed his salads and even his pellets each day to help him associate you with more “good things”. Doing some clicker training might also be a fun way to bond with him that doesn’t involve petting.
It is really hard when you have to handle them for grooming. One of my buns HATES being picked up and has a bit of past trauma associated with it so I try to minimize handling her as much as possible. Thankfully she has a short coat so she can get away with less grooming. Perhaps you can incorporate some desensitization to the brush into your clicker training? In time you may find you can groom a little each day rather than having to catch him for a full on grooming. I would also experiment with different types of brushes/combs. Buns have preferences about which they like more, so you may be able to find one that’s more enjoyable for him. For example, some buns really like the glove type brushes even if they hate being combed.
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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.