Buy a toothbrush for kids, and brush away. Make a homemade paste with water and baking soda (not harmful for dogs), and add a small amount of cinnamon if you have it (will help make his breath smell nicer).
I've been brushing my dogs teeth since she was a puppy. I use a children's tooth brush and doggy toothpaste. She prefers the minty liver flavor. Her teeth are very clean and her breath is nice and fresh.