Hello and welcome. Following on from Jane's reply, and looking at your reply to that, here are my observations. Jane is absolutely spot on in guiding you towards other observations for your bunny. It's important to look at every other visual element of your rabbits regular behavior - eating normally, looking keen for food, pooping and weeing normally, even down to his body position.
I support your view regarding emotional stress - rabbits grieve as we do. It could depend on the circumstances on the loss, but your rabbit needs to make sense and in his own way understand the sudden loss. That said, grinding teeth for grieving isn't one I am familar with. But certainly your rabbit may have some elements of behaviour over the loss so that needs to be carefully monitored.
One thing you could do is spend some cuddle time with your rabbit and gently stroke him whilst carefully checking (during stroking) for any parts of your rabbit that he might be sensitive with? Start at the head and gently work down the spine, legs etc. generally speaking if your rabbit likes the stroking, his teeth with purr or chatter (my lionhead loves this), and the response is proviked through your stroking so you can then prove that the teeth chattering is one of contentment, not of pain. If your rabbit reacts or flinches, then perhaps there is discomfort somewhere that would require further investigation.
From how you describe, your rabbit appears to be presenting as comfortable and relaxed. But there may be something like teeth or ears that might be causing discomfort and are difficult to diagnose without vetinary intervention.
If in any doubt, then I would perhaps consider route one - and take your rabbit to see your vet, explaining the concern. Your vet should be able to perfrom a health check - teeth, ears, temperature and limb checks to see if there is anything they feel is wrong.
You are the eyes and ears of your bunny - asking the forum here is a fantastic way to get support and advice and hopefully others will post their advice and help for you.
I hope everything is ok for your rabbit - losing a partner bun is always traumatic and difficult.
Craig xx
