As the PP said, cavities are caused by bacteria (S. Mutans), not food. Sugary and sticky foods feed the bacteria.
Unlike bottles, breastmilk is delivered to the back of the mouth. Baby bottle mouth is caused by baby falling asleep with a bottle in his mouth, bottles drip with or without sucking, and the milk pools around the front teeth. Unless your son is falling asleep on his tummy with a mouthful of breastmilk, it would be next to impossible to get the same scenario with breastfeeding.
The LLLI website has several links (all are short enough to read in the brief minutes your baby will give you at the computer!) containing resources, information, and mothers' stories: http://www.llli.org/NB/NBdental.html. Some important points from these links and from the 8th edition of the Womanly Art of Breastfeeding. . .
Food does not cause dental disease. Damaging oral bacteria (S. mutans) cause cavities.
Dental disease is a relatively recent event. Our modern diet contains many foods that are sticky and high in sugar (crackers, dried fruits, cereals, etc).
Genetics and some prenatal events, such as Mom's diet and antibiotic use during pregnancy, can affect primary teeth enamel formation, making them more susceptible to cavity formation. Breastmilk itself is not cariogenic (cavity causing).
Breastmilk contains lactoferrin, which binds to the iron in milk, making it available to the baby but limiting its availability to bacteria.
Breastmilk also contains antibodies which inhibit bacteria, and calcium and phosphorus which strengthen teeth.
Breastfeeding contributes to oral health by promoting normal facial and jaw development, reducing teeth crowding & potentially decreasing the need for orthodontia later on.
Breastfeeding is about the whole child's physical and emotional development, not simply about oral health.
Some mothers have found that a combination of scrupulous oral hygiene (toothbrushing morning and night, flossing if teeth are close together) and minimizing carbohydrates / "sticky" foods is helpful for preventing or reducing oral decay. You might explore the use of fluoride and/or xylitol, both of which are considered by some to help promote oral health. And, of course, finding a dentist who either supports breastfeeding or with whom you can "agree to disagree" is important!
