Not possible until he is capable of signing one. You would need to seek guardianship.
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Mr. Geffen is licensed to practice law throughout the state of Texas with an office in Dallas. He is authorized to handle IRS matters throughout the United States and is licensed to practice in US Tax Court as well as The Court of Claims.
This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
I agree with the other attorney that has already answered. If he cannot sign a legal document, then he cannot execute the required paperwork. You need to talk to an attorney that handles estate planning/probate. There are many in your area. Look on this website and on the State Bar of Texas website. I'd also ask friends and family for a referral in your area. I'm sorry that you are going through this horrible experience. All the best to you and your family.