Cavitas are not simply caused by lack of brushing. Some do get them no matter how well they take care of their teeth, some do not get any even though they don't take care of their dental hygiene. 13 is very young and many are not capable to think long term consequences in that age. For me, in that age, health care is a responsibility of a parent and child having issues preventing them from making good health choices would not be a reason for not getting them best possible health care.
By the way, since consequences or reminders don't seem to work, have you tried other techniques like token economy? I do know the family where that worked better than anything else they tried with quite disturbed children.
