My father has cancer. He may not be with us very much longer (prognosis currently unclear) and is likely to spend a large chunk of his remaining time in hospital receiving treatment. He is immunocompromised and faces a number of months stuck alone in a small isolation room, heavily monitored, with visitors unable to hug him or get close to him.
If it helps, let me give you my two pennies-worth.
A lot of men are emotionally lazy. Many are brought up like that from childhood. They get away with avoiding emotional labour in a way that women usually just don't. They are also often unfairly the beneficiaries of emotional labour on the part of women (their wives, partners, mothers, sisters etc.). So many men are used to receiving all this emotional care from others in their lives but they are not necessarily accustomed to giving it.
When emotional labour is required of men, often they outsource it to other people (their wives/partners/other family members) or they just don't do it at all. Because they're "busy with work", "busy training for this", "busy doing that". They comfort themselves with the thought that there is always more time, that they'll get around to it eventually.
It's quite stark when actually time is running out and you can't ignore it. There are regrets, yes. Looking back, I wish I had seen my dad more often. I wish we had spoken more and done more things together when he was in better health. I wish we had squeezed in just one more family holiday abroad (unlikely to happen now) and I regret all the trips and outings we talked about but never got around to doing. I wish I had got around to booking the steam train day out that he sent me the details of but I was exhausted with a non-sleeping baby and just didn't follow it up.
But these are peripheral regrets. We made a lot of other stuff happen. He was always welcome and I made time for them. I took the children to visit whenever I could. We may not have done as many of the 'big ticket' items as I would have liked together but we visited garden centres, built the sandpit together, shared takeaways, watched DIY shows together and just made each other coffee. During Covid when we weren't allowed to meet indoors, we'd each drive hours and meet and sit on a sodden bench in a rain-drenched park somewhere in the middle. My dad and I would nurse takeaway coffees while my mum played with DC. A lot of these things were done independently of my DH - he is a workaholic and finds my parents a bit much.
When your DH blames you, what he is really blaming you for is failing to take on his emotional work and nurture his and his children's relationship with his mother. When you stepped back, he may not have realised it but he had a choice - either step up or leave the gap unfilled. He chose to leave the gap unfilled and now he probably regrets it. I imagine also, mixed in with the guilt, is a fair amount of bewilderment. Like I said, many men tend to be the recipients of emotional labour. Now he is in a position where he has to provide emotional support to his mother, instead of receiving it from her, and I imagine that is hard as he is not used to this and he is feeling the burden at a time when he is also scared and grieving.
Sorry for the length of this. Just some random thoughts.