I’m very sorry for your loss. 🙁
You did everything right by letting him spend time with the body and spending extra time with him, some bunnies take loss harder than others and show their grief in different ways. Syringe feeding him is also the right move if he refuses to eat. I was going to suggest a stuffy, but it sounds like you already have that with the soft toys and a guinea pig friend.
If he doesn’t start eating within the next day or so (and keep up the syringe feeding, offering favorite foods, etc), then you might consider fostering another rabbit if possible, even if you aren’t ready to adopt or go through re-bonding just yet. I’ve also heard of people with grieving bunnies taking their bun on play dates just to visit with other bunnies. In one case, the bun would only eat when another rabbit was present, even though they weren’t bonded.
How are his poops? If he seems to be really slowing down you might also need more treatment (like pain meds, etc. if he goes into stasis). You might give some tummy rubs and simethicone along with syringe feeding just in case he’s a little gassy.
Again, I’m really sorry for your loss.
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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.