Well, first of all, find yourself another pediatric dentist. This guy doesn't have a clue how to work with children. As soon as it became obvious that the whole procedure was going to be a battle, he should have come up with a better way to handle it. The constant drooling needs to be checked out by ANOTHER pediatric dentist or by a physician. Increased production of saliva is a common reaction to something foreign in the mouth, but the body doesn't usually interpret a cap on a tooth as a foreign body because it should just feel like any other tooth in the mouth. Given that he struggled the whole time, I would be concerned that there has been damage to a salivary gland, a nerve or some other part of the mouth. I'm sure that part of what is going on may just be psychological from the trauma he experienced, but do consider possible physical causes for the drooling.
I am so sorry for your son's experience. No child should have to be treated in such an inhumane way. Anyone who works in pediatrics should have had a better "plan B" for dealing with a struggling, fearful child than a papoose board and should have used it much sooner than 5 minutes before the procedure ended.
B. V. (long time pediatric nurse)