How to establish brushing

Make it fun and don’t rush into it. Start with books about oral hygiene. Make some fun activities such as - brushing dinosaurs’s tooth with disposable toothbrush (you can buy those salted egg that is covered in soil). Also can paint egg cartons with white paint and toothbrush to mimic toothpaste. When he is ready, both of you brush together (don’t help him), let him get used to the idea of brushing every morning and night before you teach him the proper / correct way to brush.