Maybe your child doesn't want to brush under normal circumstances, but perhaps a Superman cape will change his mind. Or, he can help Luke Skywalker (or another favorite character) battle evil by destroying tooth monsters using his toothbrush-shaped lightsaber.
You know your kid better than anyone, so venture away from reality and use your imagination to create stories and scenarios that will inspire him to tackle those teeth. Cavity monsters are a real threat, even if they're not as exciting as the stories you'll create about them. If storytelling isn't your forte, you can find plenty of free, interactive tooth-brushing tales online, and books on the subject are available at your local bookstore and library.