J.M.
Well he probably only goes to the dentist every 6 months. He may not even recall much of the past vists. In order to get used to the dentist you may need to take him for visits often and practice x-rays at home. Just do the things they do for x-rays. Have him bite down on some plastic thing to practice and then pretend to x-ray. You can practice having him open his mouth for the dentist at home. I think it will take a lot of practice and a lot of friendly dropin visits to the office.
Good luck