Teaching an autistic patient to do anything depends upon the level of function of the child.
High-functioning autistic children have no problem learning to brush their teeth and learn at about the same rate as any other grooming habit you might teach them. Lower functioning autistic children may need your help 2 to 3 times per day to brush their teeth.
I am not an expert on autism; however, I have had a number of autistic patients, one of whom is in his teens whose mother still brushes his teeth for him. He cannot vocalize clearly, and is moderate to low functioning. I provided his mother with a mouth prop to insure her safety while brushing and flossing his teeth. She sits on the couch and he reclines with his head in her lap with a table lamp shining light into the mouth. She brings him to my office as often as possible for regular cleanings and check ups. She is extremely dedicated and has prevented him from getting dental decay except for one filling which I perfomed with her assistance in my office.
If there is any question that your son might hurt himself while attempting to brush or that he may not cleanse the teeth properly, you need to assist him and brush and floss for him. If there is a threat of getting bitten, only floss with a long, "Y"-shaped floss threader and/or use a metal covered in rubber mouth prop (purchased thru a dental supply house).
Remember that nutrition plays a huge role in oral health. Limit liquid and solid sweets. Make sure the diet is rich in fruits and vegetables and a source of calcium and vitamin D, such as milk and cheese.
You would be surprised how much special needs persons can learn with a little effort from their caretakers. I would certainly give it a try. I also recommend finding a dentist who works well with special needs patients and make an appointment for a check-up.
Update, check out this site for more information: http://www.nidcr.nih.gov/OralHealth/Topics/DevelopmentalDisabilities/DentalCareEveryDay.htm