Is there someone who can go with you, for extra support?
Perhaps, call the Dentist,...and give them the head's-up that your son is scared to go to his appointment today... maybe that way, when you get there, they can "assist" somehow...
Yes, lots of Parents tell their child of some reward or treat or "prize" to give the child an incentive for the whole ordeal...
What is something he REALLY likes or wants? Perhaps, reward him with this... since he is so frightened....
ALso, our girl used to take a favorite "lovey" or stuffed animal/doll with her to her appointments... it helped her to feel better.
After the appointment, make sure you really talk with him about it... tell him how proud you are, how "brave" he was... and what not. Assure him, and have a special time with him afterwards....
All the best... I know how you feel.. my daughter used to fear the dentist too... so my Hubby would take her there... since he was better at handling her and her crying/screaming at the Dentist office....
Big hugs to your son... I remember my own fears of these appointments when I was a child...
Take care,
Susan