There are plenty of ways to make your child more enthusiastic about brushing. You can take an educational approach by showing him videos of the benefits and importance of brushing his teeth. If he is comfortable with his dentist or dental hygienist, you can also have them explain it to him during his next visit.
Or, you can take a more interactive approach and show him that brushing his teeth is important with demonstrational toys and instruments. You can purchase these online, they are plush toys that are used in some offices to show kids how to brush their teeth.
I hope this help and thank you for your question.