Dog's Dental Health
The ASPCA recommends using baking soda as a DIY toothpaste for your dog.
Get yourself a toothbrush made especially for canines or a clean piece of soft gauze to wrap around your finger.
Ask your vet for a toothpaste made especially for canines or make a paste out of baking soda and water. Never use fluoride with dogs under six months of age. It can interfere with their enamel formation. And please do not use human toothpaste, which can irritate a dog's stomach.
Special mouthwash for dogs is also available. Ask your vet.
Here is a link to the article:
https://www.aspca.org/pet-care/dog-care/ten-steps-your-dogs-dental-health