If he is still mentally competent and can communicate his desire to sign a power of attorney (even if he can't personally sign), he can execute a power of attorney. Hire a trusts and estates attorney to help you: the lawyer can draft the agreement and make sure it signed and witnessed properly.
However, if he is either not mentally competent or wholly uncommunicative, he cannot grant a power of attorney: for a POA, it is necessary that the person be sound mentally and be able communicate their agreement or consent to the POA. In this case, you'd have to bring a legal action to have him declared incompetent by a court and have someone appointed his guardian. This is not an easy action to bring; speak to an elder law attorney about helping you.
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