Show him this from the American Academy of Pediatrics - http://www.aap.org/publiced/BR_ToothDecay.htm:
How can I help my child prevent tooth decay?
Tooth decay develops when a child's teeth and gums are exposed to any liquids or foods other than water for long periods. The most common way this happens is when parents put their children to bed with a bottle of formula, milk, juice, soft drinks, sugar water, or sugared drinks. It can also occur when children are allowed to drink from a sippy cup, suck on a bottle, or breastfeed for long periods during the day or night.
To prevent tooth decay:
- Never put your child to bed with a bottle or food. Not only does this expose your child's teeth to sugars, it can also put your child at risk for ear infections and choking.
- Only give your child a bottle during meals. Do not use a bottle or sippy cup as a pacifier or let your child walk around with or drink from them for long periods.
- Teach your child to drink from a cup as soon as possible. Drinking from a cup is less likely to cause the liquid to collect around the teeth. Also cup cannot be taken to bed.
- If your child must have a bottle or sippy cup for long periods, fill it only with water.
How to clean a child's teeth
Keep your baby's mouth clean by gently brushing the gums and teeth with water and a soft infant toothbrush or gauze. Once your baby has 8 teeth, you can start using a child-sized toothbrush for daily cleanings.
Brush your child's teeth 2 times a day. Start by using a fluoride-free toothpaste. When your child is able to spit and not swallow the toothpaste (usually around 2 to 3 years old), you should continue brushing his or her teeth using a pea-sized amount of fluoride toothpaste.