Well, in this case, when addressing directly to him, usually it's just "доктор" (no name attached).
If you know him well (he's your attending doctor) you say Сергей Владимирович.
And if he's your good fellow and you are about same age you can say Сергей or Серёжа
Господин\Госпожа can be heard in media, political debates, official papers etc, but common people rarely use it, mostly with irony\sarcasm.