I can imagine ho you are fretting, but to be honest, if he has cut ties with his family, it really isn't your concern, considering he has cut ties with you too. It's obviously something HE feels very strongly about and as such he has had time to contact people if he wanted them to know about his illness.
It seems such a shame he is going to fade away and die with no one by his side, or to hold his hand, but that seems to be the way he wants it. Respecting that may make you feel guilty, but really you have nothing to feel guilty about, just a very sad situation to come to terms with. It is his wish.