OP, you have a job to do. People will be coming and going all day in a doctor’s office. The office door is presumably unlocked. You are worried about something that should have been of concern before you took this job.
If the doctor is a yarei Shamayim (not Yiras Shamayim; he HAS yiras Shamayimh if he is a YAREI Shamayim), then he, too knows the Halacha. Speak to your own Rov if this is really an issue for you. Otherwise, be a professional, act professionally at all times, and schedule patients at times that allow you to feel comfortable that you will not be alone for extended time with him. All this goes out the window if he were to act or speak in a way that makes you feel uncomfortable. Then you leave. JMO.