The trick with this is to use a dog toothpaste. These are flavored to taste nice to the dog. Teach the dog to allow you to brush the teeth in stages, starting with licking toothpaste from a finger. Practice handling the dog's mouth, praising him when he behaves. Then rub some toothpaste on the gum, and slowly build up to using a finger-toothbrush to rub the paste over the gums. Once the dog allows this you can start using a dog toothbrush.