Rebecca, we are going thru this right now, coincidentally.
DS has always been "orally defensive" and I knew the dentist was going to be a challenge, he's 6 and we just took him for the first time a couple of weeks ago. We tried a regular dentist, and he wouldn't even open his mouth, and she kinda freaked- well not too bad, but she just didn't see any way to get anything done-
We called up the big children's hospital two hours away, the same one where we did his original diagnosis and his asthma stuff and all. They have a dental clinic there. I love those people. Because they will never say "We don't know how to deal with this", they just figure something out.
We had one appointment where the dentist managed to pry his mouth open enough to discover several cavities. We go back in about a month. They are going to give him Versad (oral sedative) low-dose just to chill him out a little, then nitrous. (the mask won't be an issue since we have a nebulizer he is used to) The dentist said with most of the kids w/ autism he's had, that works. If not, they will have to put him under a general and do it in the OR.
It's been hardest on me, I think, just another one of those things, where it hits you that things are just not going to be normal.
But anyway, I would suggest calling the people that did his diagnosis, if you went to a specialist, and ask for recommendations. Or if there is a big children's hospital/ clinics around there, to call them to ask.