You should speak to the dentist first of all and tell him how you feel. If he doesn't take your concerns on board then go to your manager and request to be swapped with someone else or have a rota system so he has a different nurse. Let your manager know how he is making you feel and that you tried to speak to him. However, I would personally have a chat with the dentist myself and see if that improves anything. He may respect you for it. I've worked with difficult principles and hated it, now I am great friends with them.