My dd has dental phobia which started about that age with a cracked tooth.
So my advise is take it slowly. Bribery (what you'll do after as a treat/day off school if that's his line), threats (if it gets infected the pain is huge and then it will have to be done) and let the dentist know in advance that he's really nervous.
Depending on him, will be be better if everything is explained (like my ds) or nothing (like dd)? And let the dentist know.
It's taken over 10 years for dd to get over her phobia, and she isn't yet, but she is much better, mostly because the dentist has been brilliant, taking it slowly and letting her take the lead when possible.
I remember an early trip. The dentist said, "I can fill this now, or you can make another appointment and come back". She was out of the seat in a flash saying she'd come back.
I barred the way and told her she'd feel better for the next 5 minutes, but then would have a week (or longer) of dread and still have to do it. She did come back, and we managed to get her through it.
Would that work with him: tell him it will have to be done, and the waiting is the worst part?