I was recently appointed as Conservator for my 70-yr old father. He has advanced dementia & had no financial or health related documents (POAs, DNR, living will) prepared in the event that he could become incapacitated. We live in California - can I prepared an Advanced Health Care Directive & DNR on his behalf - the family is aware of his wish to not be kept alive on machines.
I suggest a POLST rather than an DNR. The POLST goes into greater detail and is more comprehensive.
The important thing is the document must be kept with him at all times. A copy should be in view in the home or in his room if he is in a facility.
If paramedics do not have the document then as far as they are concerned it does not exist. If they are called they will do what they have been trained to do unless there is a document to tell them otherwise.
I am sure you can print out a POLST for the state of California. Most are typically printed on a Neon color paper because it is easy to see. But it is not required.
A doctor will have to sign the document.
https://oag.ca.gov/sites/all/files/agweb/pdfs/consumers/RWJRAEL.pdf
Of course the only difficulty with the website itself is, alas, that it's aimed at people who are planning their own care. Ain't hindsight a wonderful thing? But your father is lucky that his conservator is so conscientious about taking his own stated wishes into account.