What you are going through right now with your partner is really hard going. I'm glad you've reached out for some support and different perspectives and experiences to maybe help you along your way.
Croix has brought up maybe the most important point: Take Care Of Yourself. You must. Before you can take care of anyone else.
My partner is going through what sounds like a very similar situation to your partner: her mother passed recently & she has been in deep grief, after a very tumultuous and difficult relationship with her mum. We have been talking about it, and wondering if the grief she is feeling now is a lot of old wounds from over the decades that have been pushed down until now. It may be something that your partner may be experiencing as well.
From how things have been in our household for the last little while, i would suggest trying to mentally extract yourself from the turmoil &, if you can, from thinking it's your job to take his pain away: it's not. If you can just be there for him, be that constant, steady source of ok-ness in his world, that is what he needs. I know he is pushing you away, & that's awful (i know), but these could be (dysfunctional) coping mechanisms he is using.
In saying that, it's not ok for you to be used as an emotional punching-bag, so do take care of yourself and remove yourself from the situation when you need to.
My partner has said some things recently she hasn't meant, simply because she's confused & in emotional pain.
If you can make some allowances, it may help in the long run, little household tasks etc that normally i would expect my partner to help out with, I've taken them over for now. Not to make a lifelong habit , but simply to survive this period.
When i had some grief counselling several years ago, one of the best pieces of advice i received was to *not make any big life decisions whilst in the depths of your grieving process*. I know your partner has said he wants to separate and now you are in different bedrooms - do you think there's a way you could bring this up to him saying that you are there for him, you understand he needs space right now, but maybe any final decisions can be left for a while?
Intense emotions can really cloud our judgement & if there's scope for giving this decision some space & time, it could save a devastating separation process with many regrets later. Do you think you could gently suggest something like this to him?
This is really difficult & painful time, but his grieving period won't last forever. You being there for him (within reason), is one of the biggest gifts & acts of love.
But please do take good care of yourself, because right now, you are taking over some of the relationship reins for both of you.