This was totally uncalled for, and he needs to be reprimanded by the ADA for his handling of your son. There are other angles you can take here, but I think you might have better luck with the dental board. He is totally UNPROFESSIONAL!!! in his manner, and may complicate your sons condition??? For that alone, I'd be willing to go and punch his lights out!!! That's what he needs.
Also, you can report him to your ins. co. and tell them what went on. They can do what's known as "black" listing and refuse to let their clients see him. That will kick him where it hurts!!!

Please do not ignore this, your son was a victim here of extreme mental cruelty. Even abuse........