Legal Question in Real Estate Law in California

It must be done in the state where the person granting the power resides, under the law of that state. The inability to physically sign is not a big issue as long as they can confirm their assent. You will have to review the nature of the physical incapacity with an attorney to determine the proper method to authenticate consent. The far bigger issue is that you mention dementia. If the person is not fully mentally competent at the time of granting the power, it is voidable and all acts done under it may be challenged and set aside. For that reason, many third parties may refuse to acknowledge the power of attorney. If there is any doubt about the person's mental faculties, you are much better off seeking to be made a conservator of the person's estate, rather than trying to go the short-cut route of a power of attorney.

You probably don't. If he is not 'legally competent' he can not validly grant a power of attorney, and no Notary would endorse it after observing he was not competent. You have clearly described his condition as 'legal incompetence'.

If that is so, someone needs to seek court appointment as a Guardian or Conservator. That must be done in the Probate Court local to him. The local court would hesitate to appoint anyone other than a local resident, unless the incompetent person were in a secure care facility properly supervised.

If serious about hiring counsel to help in this, and if this is in SoCal courts, feel free to contact me.

Although the law (i.e., judges) will let a person sink pretty far into incompetence before finding him or her sufficiently incapable of handling his/her own affairs to invalidate his/her acts, such as giving a power of attorney, you are much safer yourself from allegations of elder abuse or other court actions if you apply to the court for a conservatorship. A power of attorney from a person of questionable competence at the time of granting it may in turn be of questionable validity. You may have heard of incompetent people having their affairs managed under a power of attorney, but these are "durable" powers of attorney that were granted while the person still had full competency.