COUNCILS
Brushing your teeth is an action that you have to learn and get used to:
1) Let your dog taste the toothpaste first, so he gets used to the taste
2) Get your dog used to an "object" on the teeth: rub the teeth with a finger. Then repeat the operation with the toothbrush.
3) Once your dog has gotten used to the steps mentioned, you can start brushing his teeth.