Brush his teeth. "Dog toothpaste" is mint flavored, but you can get toddler's toothpaste. It's not minty, and it's safe to use with dogs as it doesn't foam a lot and it doesn't matter if he swallows it. Use soft toothbrush, and only do it for a second or two at first if he doesn't take to it.
On our dogs I use a dog mouthwash type thing we got from the vet. You just squirt on there teeth and rub there teeth a lil. Its fast and easy. Also there a spray you can get for dogs and cats called plaque away.