How long can a person survive drinking only a small amount, mere ounces a day?

My stepfather has Parkinsons and has went through periods of time previously when he stopped eating and drinking and had hallucinations. He is doing it again but this time he is really deteriorating. He is living at home with my mom but she and some help from hospice is caring for him. She is 84 as is he and is not in great condition herself but is doing her best to keep him clean and well cared for. He is sleeping most of the time, is skin and bones, not eating and drinks only a sip or two during the day. It has been 21 days of no food and very little water but hospice nurses say his vital signs are good. Is it possible he is pushing this to the limit because of some mental issues and may still survive? I don't see how, seems to me he is going to far this time and yet I have seen him come out of it when almost this bad. I am worried about my mom, she is getting worn out and I worry she may die before he does just because of her over doing caring for him.