I don't know how sever your child's Autism is, Julie, but my son's is severe. Luckily, his uncle is a dentist so he allows me to come in and facilitate a successful exam.
There is not much you can do to calm the child down if he doesn't like people looking in his mouth(who does?). You can have your Pediatrician prescribe some Chlorohydrate(sp?) to be administered prior to the exam, but that doesn't always work.
What I do is straddle the dental chair facing my son as we begin. Sometimes the pressure I put on him is relaxing, but it is mainly to give me a postion to hold him still. Then I have to try to keep his head still. The dentist has a tool that will spread his mouth open and lock it in place. This will have to be put in and out a number of times throughout our exams. Put some moisterizer on the child's lips because the spreader can make them sore.
It's not an easy thing, but like most difficult things, such as haircuts and fingernail clipping, it can actually become a relaxing event that a child even looks forward to. It's hard to believe, but it's possible. My child now loves to have his hair cut and manicures are a favorite. In the beginning, these tasks could be very difficult and could lead to quite a scene.

Oh, make sure Dad is going.