We just did this with our daughter. She did really well until the x-ray part. She got the fronts done, but the sides hurt a bit so she cried and then the appointment was pretty much over.
See if they can do the x-ray part last. Also, we practiced a little before hand. I would look in her teeth with a toothpick, a popsicle stick, and mirror. We'd clean/floss her teeth, and then she did me. We used a towel for our bib and we even did x-rays. It was fun playing that game, and it prepared her for how it would be at the dentists office.
I remembered when I was little, always getting to go to a donut shop after the dentist and getting to pick out a huge donut. (Which is kind of wierd when you think of it...) But, I continued that with my daughter and she loved it. She had a great experience. We never get donuts, so it was quite a treat for her.
It doesn't have to be a donut, but some other treat/reward that you can continue might be a fun little incentive and something that your son can have a choice in makes them feel like big people.