Definitely talk to the dentist personally, not the receptionist. I have the same problem and my son is 20, he is just so afraid. But I learned my lesson when he was young, if they say you cant go back with him, leave the office. They wrapped him up so he couldn't move, found out after because he was all red and sweaty.
So to say, I never took any of my kids to that pediatric dentist.

It also helped my daughter to have a private room rather than just the open space she feels more comfortable this way. Not sure if the dental clinic you are looking into has this option or not.

Definitely the dentist, may be able to adjust or he may know other dentist whom he can refer, in some states medicaid will pay if there has to be a referral.

We are very fortunate, our dental clinic does very well with children with special needs and kids without, but it took my 9 year old daughter 4 1/2 years to be comfortable enough with him to sit in the dental chair and let him look at her teeth. She has just started becoming more verbal and has said that the face mask they wear scares her, the tools that they use scare her, and it is better if she knows what they are going to do with the tool before they come at her with it. They even explain the mirror to her or she will move away until they tell her what it is and what they are doing with it.
Before she told us all of this she would sit in the corner and scream/cry the whole time he just looked into her mouth. I understand how stressful the dentist can be for the parent and the child, good luck:)

I would talk to the dentist ahead of time as someone else said as well.

I always talk to the dentist ahead of time, explain my sons autism and ask for extra time to be scheduled in our appointment so that we can take things slower