Most likely under all of the anger that your husband has expressed over the years is a profound sense of sadness, depression, and loss regarding his "horrible childhood." I don't know what the details are, but I suspect that the potential "literal" loss of his mother is triggering earlier feelings as well. Think of it like an onion, i.e., when you peel back the skin and go through the layers it goes deeper and deeper. Very often anger is a defense against other feelings, particulary in males. Maybe your husband felt powerless as a child and his mother didn't protect him. Perhaps being involved with her and her treatment now will empower him and finally help him to heal. In any case, I think that letting him express himself to you and just validating his feelings will be helpful. But it may not be easy for you so make sure that you have an outlet for yourself.
I'm not sure if this is helpful at all. People often feel better just from having someone else listen to them without judgment.
Peace to both of you.