Always Dr. followed by his last name. He also goes to the public pool, as do I. When I see him there, I always of course give a hello, and address him the same. I feel awkward not doing so. However, I don't know if it's okay to use just his first name in a public setting and don't really want to ask. I just want to be respectful and proper.
Yes. Thing is, while there, or in a public setting, and not in his office. I do see and hear others call him by just his first name, which makes me think maybe it is okay. However, I do not know how well they know him outside his office.