We have elderly, single dental patient with signs of dementia. How do we help him to get a legal guardian?

As said APS, Adult protection services. Have the Dentist call, more clout. He can ask for a well visit. Also, telling them what he has seen and experienced.

Did the man put an emergency contact on his paperwork? If so, give their name to APS.

I would call adult protective services in your area and explain what you did here, ask what options there may be. At the least they may do a wellness check and contact family.

You need to report him to adult protective services for your county. Once there are a certain number of reports, it triggers a search for extended family. If none, he can be placed under the care of the state. The state pays professional guardians to make sure these folks are in a safe place and their funds are used wisely, with lots of state oversight. If one has no family or friends to step in, it's a very good option.

My mthr separated herself from me (only child) but when APS talked to her, they were able to piece together enough information that they found me & hubby 2 states away. We rescued her. Some would have thought she had no one by talking to her, but they probed. An estranged family member may be out there waiting for their loved one to define to the point of accepting help. 💌