Are there special forms for a parent of a disabled adult to show to health care providers?

My parents have a disabled child/adult who is 40 years old. He contracted viral meningitis at 9 months old and was left physically/mentally disabled. He is having some medical issues and my parents are constantly asked if they have power of attorney over him. My understanding is that a power of atty only works when the individual had capacity and could sign the form. But my brother never had legal/mental capacity. He''s essentially a 9 month-old baby in the body of a 40 yr old. So what are these people asking for? Also, these people ask my parents for my bother''s ID. How would they go about that? Someone told me I could just get a notarized letter stating he is disabled and my parents can make decisions for him. Is that true?