You can call emergency services for a person who is a danger to himself and others. Let them know what to expect and ask if they can take him to a facility that offers mental health treatment.

You might also speak with the police department to ask how to get help for someone who can't do that for themselves.
Also, Health and Human Services would be an agency to contact. I know of a person who was 'reported' as neglecting herself and Health and Human Service intevened.

You are very kind to want to help!