It's hard to tell if your son will have any decay or tooth ache because everyone is different and some people are more prone to dental issues than others. The only way to truly know is to take him to a dentist. I would research in your area and see if there are any dentist's who work with Autistic children. Some dentists specialize and will be more patient and willing to work with your child if they are not following directions.
In the Son-Rise Program playroom, we would work on eye contact, interactive attention span and flexibility before introducing the idea of toothbrushing, then we would incorporate it into our games once the child was connected to us for a longer period of time. We role-modelling how fun it is to brush our own teeth, fill the playroom with several different fun toothbrushes and bring the toothbrush into our interactions throughout the day. For example, if your child likes music, we might play a drum with a toothbrush and when they are motivated, offer it to them to play, and then graduate to them brushing their own teeth.
Keep up the great work!
Becky Damgaard - Son-Rise Program Teacher