This is quite complicated and is very case-specific to the individual and their plight. It is not a conscious decision to shut down. In fact, the conscious mind might well be fearing loss of function cognitively, in seeing the signs developing that the mind is slipping and memory is less and less reliable. This can cause great fear and consternation and dread as well. Nonetheless, it often is progressive and unstoppable. The decision is made deeper within, by the deep subconscious typically, as it bears the greatest brunt of stress from all of the conflicts people become involved with in their state of limitation, causing great difficulty with being successful and accomplishing achieving a state of inner balance and harmony, staying in divine alignment, and seeing a robust healthy body flourishing, to be enjoyed and reaping the harvest of benefits it brings.
For most people, life is complex, intermittent in its blessings, if any, with much uncertainty, doubt, fear, and endless worrying about the future with a state of chronic anxiety that becomes too great a burden for some, and that is the reason for the decision to leave, in a sense, by shutting down the body, shutting down the awareness, and what happens is the deep subconscious wants reality to be blotted out so that it can finally get some rest and inner peace. It has the power to manipulate the body in various ways, and that leads to a kind of inner avoidance of incoming stimuli, and that neglect can lead to a decline in actual function—the ability of the nerve connections within the sensory input to trigger memories and thoughts that can be acted on in meaningful ways, as happens normally. That disruption causes a kind of atrophy, much like becoming sedentary, and then the muscles waste away because they are not being used and the body will begin to conserve resources and not replenish them. The same happens with brain tissue and function and this is why there will eventually be pathologic changes within the brain itself with the typical dementia disorders. They are not the cause of the dysfunction, but are an indicator there has been a self-imposed neglect of duty, so are a useful sign as a diagnostic indicator but not truly representing an important mechanistic aspect of the functional decline, and we have commented on that elsewhere.
So your father is in the grip of this dilemma, having set in motion his undoing, cognitively. This is not something you can talk someone out of and the unfortunate reality for him, it is an incomplete escape because his mind is still present, he simply cannot communicate to you the fact that he still thinks and is aware of things. We say this to you not to torment you further in seeing his situation to have additional unpleasant aspects, but it is the key to what you are asking in terms of what you can do to help him. The first thing we would say is all of your care you are giving to him is noticed and greatly appreciated by him personally and recognized as such within his consciousness, which does persist and exists in a functional state, but he is locked in with the true sense of that syndrome representing consciousness trapped in a non-working body thought to be essentially in a vegetative state, so the lungs and heart are going but the mind is gone, when such is not the case.
You can talk with him and you can show and tell him that you have love for him and he will benefit greatly from these gestures of loving concern. We understand the relationship has had its strains and he may or may not appreciate his role in that, but he is in no position to complain about you, as you are his only link to the world in a meaningful sense. Anything you do will be meaningful because he is not absent, he is not checked out, he is simply unable to make his body respond to his consciousness. The brain is not functioning adequately to form thoughts and to create speech and body movements and body language to act like his normal self. The only other additional step that can be taken of any great benefit would be to arrange for a session of Deep Subconscious Channeling with Trauma Resolution to assist him to resolve the traumatic situation he faces with a decline in cognitive function—that will most likely be linked to prior events in other lifetimes when he experienced loss of some kind, and that would help ease his suffering currently.