I don't blame you - I would have been angry too. What a way to set up a child for a potential life-long fear of the dentist (like they need that!).
If it were me I would take a picture of the injury, just in case there are any further problems (not that there would be necessarily). Also, I would wonder if I should see my ped (depending on how bad the bump is).
I *would* be careful how I reacted in front of my child. If he quickly gets over it and forgets about it - great. Hopefully that will be the case.
Good luck to you - sorry this happened! We moms don't need that sort of thing!