I'm can relate to your experience both as a mom and as a dentist. When my oldest son first started seeing the dentist--age 3.5, I sat in the waiting room and enjoyed reading my book (i.e. successful appointment. The next appointment we had to reschedule (nothing accomplished), and at the rescheduled appointment, I was in the room at the foot of the chair; he was anxious but we got through it.
As a dentist, I see a lot of kids, and it's not uncommon for kids to be a little daunted by what's going on. A lot of times the chair and lying back in it are the anxiety producers; sometimes it's the water and air sensations. I have found that some kids do better by sitting in a regular chair (the dentist would have to stand) to be treated, or even by standing up while being treated. (I've even treated the kids while they sat in their stroller--wherever they are comfortable!) Also I'll have the kids participate by holding the prophy paste or the suction...they like having a little control. Sometimes it just takes a little creativity, but usually the treatment is accomplished. Eventually most kids will become acclimated to the chair. With the water and air, your dentist could try get letting your son rinse with water from a cup and spit in a sink. Again a little flexibility can go a long way.
Good luck, maybe some of these ideas will work, but remember over time it should get easier for your son.