This is really difficult. Going got play devil's advocate a bit here...on the face of it the son sounds like a selfish pig, but you don't know what the history is. Perhaps he's tried over the years to change her situation but she will not allow him? The elderly can be very proud and stubborn, to the detriment of their own wellbeing. And I know he doesn't visit anywhere near enough, but again, how can we know if this is 'fair' given the lack of background? Maybe they have a slightly estranged/strained relationship and he only comes occasionally out of duty? I know I would not have been any different with my father, no matter how in need of care he was, as he was always pretty much estranged to me when I was growing up and in adulthood I felt I barely connected with him.
Let's say hypothetically, old lady has done equity release and blown all of her money so there is no value left in her house, and she refuses point blank to go into a care home, because she only wants to live alone, or with her son. Perhaps her son cannot afford to keep her, maybe both he and his wife need/want to work, and do not have the space for her. Or perhaps she has money but refuses to use it, and he is willing to help her, but she must be prepared to sell her home to fund changes to the situation - after all, why should he fund it?
Perhaps she has been the MIL From Hell and the son's wife would rather stick needles in her eyes than have the lady to live there. Perhaps he only visits infrequently because he is tired of banging his head against a wall with her and he finds it maddening to have to listen to the guilt trips and yet be powerless to do anything practical to help her that doesn't cost him money, or that she will actually agree to.
On the other hand, he could just be a selfish git....
But in the meantime, she needs more practical support and so I agree with everyone else - call an ambulance next time, and SS, and make it clear that she is unable to live independently any longer and that will force their hand one way or another. Can someone refuse to go into care in these situations? I'm not sure.