Remember, many people with Down syndrome cannot breathe well through their nose. Sorry to put this near the end, but in my experience this only works on mildly frightened patients. I have not tried hypnosis, acupuncture or biofeedback because I suspect that they won't work reliably in most patients. If your dentist is now doing this with great success and doesn't need any other assistance with your disabled child, give him some cookies next Christmas. There are better techniques.
Restraints are okay if the child simply needs reminders to keep hands away from mouth, etc.
Go in the operatory with the child and watch the procedure. You can leave and wait in the reception room if you trust the dentist. Both children and disabled people of all descriptions tend to avoid telling parents when they have toothaches. All people with teeth need brushing. Brushing followed by flossing is better if possible.