Capacity in not an all or nothing determination. A person can suffer from diminished capacity and still make or revoke a POA. Your uncle should be able to revoke the POA anytime before he is adjudicated as incompetent by the court. The consequence of a person with diminished capacity revoking their POA is often that the attorney-in-fact petitions the court to have the person declared incompetent and a guardian/conservator appointed.
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You describe a regularly occurring and difficult situation: a loved one appoints someone they trust to make decisions for them if they can’t, then, when it may be time for that medical power to be activated, the "principal" (your uncle in this case), is not ready to let go.
The situation is one that can result in tragically damaged relationships if not approached with care. In the right environment, the family can have a loving, yet frank, conversation with your uncle to help him consider the options and make a decision. Your uncle may acknowledge that he is becoming mentally tired (a normal aging process), but that he is not ready to give up total control. Working with him to decide what control he can give up now.
If the doctor is of the opinion that your uncle should not live alone (this is different from making major decisions), then you can discuss the alternatives: Independent Living, Assisted Living, Skilled Nursing, Nursing Home. The range of independence and assistance options available should be sufficient so that your uncle can ease into assistance at his pace, rather than feeling pushed into the “nursing home”
If either the family or your uncle becomes frustrated, and the relationship deteriorates, your uncle could cancel the power of attorney. The family could go to the probate court and have a guardian and conservator appointed for your uncle. That decision brings its own set of challenges, which may or may not be worse.
I recommend that you, your uncle, and those trusted family members meet with a skilled estate / elder law attorney to review your uncle’s options and talk about the aging process so that you can hopefully minimize the potential struggle for control and maximize the power of family helping family during those times of need.
Disclaimer: My answer is provided without all relevant facts, let alone your unique objectives. No attorney-client relationship is hereby created as a consequence and you should not take (or not take) any action based on my answer. I highly recommend that you consult with competent legal counsel before deciding to take or not take any action, as every situation is more complex than it appears.
The healthcare power of attorney only gives the agent power to act when the grantor of the power is incompetent or unconconcious. So the short answer is the agent could not Make him do anything until he is declared incompent by a court of law.