"I looked him dead in the eye and asked him if he seriously expected me to comfort him through MY mother’s diagnosis. He just threw a tantrum."
I think that tells you everything about him, as a person, I'm afraid, OP. Your husband reacts like a child might - or a self-absorbed teenager. Not how a man, a husband, a partner is meant to. All of your attention and focus is meant to be on him, and no one else. The older your husband gets, too, I'm afraid the worse this will become...
It's also not just men who do this. Other posters have referenced their MILs behaving like this - and I can add my own mother to this. This year, my teenage son had to have a large, potentially nasty growth removed from his face, surgically. Now, we were very fortunate in that it was benign - but there was a strong chance it might not have been. He's 14, realised who his friends were/weren't, held his head up throughout the whole ordeal... but he was, understandably, completely terrified (as was I, his Mum). My mother...? Sat in the hospital waiting room, having given us a lift over, carrying on about how painful her finger was. She had a hangnail, but the way she carried on, it was as though she expected... well, no; actually, I know she expected me, my son, the nursing staff, and any passing consultants to swoop in and fuss around her. Not my son or any of the other anxious/ill-looking people waiting for their appointments to be called. It's a compulsion, really. A form of attention seeking. Now, my son and I just ignore my mother's wittering on... but we don't live with her, whereas you presumably have your husband's attention seeking 24/7.
for you and for your Mum. I hope her treatment is successful, and that you figure out how to ignore your husband - and, more importantly, perhaps, that your Mum does, too. Shame on him for behaving like a child, when you both need him to be a man. 