My autstic 8 year old is getting anesthesia for dental work in hosptial. How safe is it? I'm hearing about a girl brain dead from routine surgery.

Hospital anesthesia. General anesthesia and IV sedation are very safe procedures when performed by trained anesthesiologists, especially in a hospital setting. Yes, all procedures have risks and you should be fully informed of the risks. If my child required it, i wouldn't be unduly concerned. Having said that, a plane or car can crash into your house, and people cay die from penicillin, peanuts, or a bee sting.
Generally. Very safe. Talk to the anesthesiologist to address any concerns. They will do a work up on your son to be sure there are no hidden ailments or drug allergies. Whether general anesthesia or sedation talk to the anesthesiologist.