Another option besides switching doctors is to politely educate!
My son is 2, and at his 18 month dentist checkup there was some decay in-between his front teeth in about 3 places, not severe yet but will probably have to be filled. Breastfeeding during the night (maybe once or twice a night at that age, depending on teething/travel) was blamed. This particular dentist's website has a lot of misinformation about breastfeeding, including that baby bottle tooth decay "is a pattern of rapid decay often associated with prolonged nursing, bottle-feeding, etc." and to stop on-demand nursing with the appearance of the first tooth (which was 3.5 months for my son...). They were very "nice" about it but did attribute it to breastfeeding at night. Recommended night weaning or wiping/brushing teeth after every night feeding.
We did mostly night wean since the decay had already started, and for other personal reasons. However, in the absence of any dental problems, I don't think that breastfeeding at night itself causes cavities. This time around (expecting second child soon) I will be night weaning when we're ready, but doing the following things differently:
1. Vitamin D supplements for myself during pregnancy and nursing. I believe I was low during 1st pregnancy and this is known to increase the chances of childhood tooth decay. Vitamin D helps bone/tooth formation and also helps the body fight infections, including the germs that fight tooth decay.
2. When solids are started, I will brush baby's teeth very carefully before bed so no food particles are left. We did this somewhat, but next time I will be more vigilant.
3. I will start flossing earlier, especially if baby has any teeth touching each other. I didn't know it was possible or necessary to floss a 1 year old (now I use the P-shaped plastic flossers to floss my son's teeth), and the decay happened where the teeth were touching each other. I think food particles go stuck there, and combined with the breastmilk overnight, caused decay. I think that less saliva is produced overnight which is why night-time is of concern more than daytime....not sure.
4. Use xylitol and/or fluoride toothpaste, and xylitol candies for the whole family to reduce the amount of bacteria in our mouths. I know you're not supposed to share utensils, glasses, etc. but that just is about impossible to do for us. Sharing happens. I'd rather try to reduce those bad bacteria in all of our mouths.
In the 6 months since we discovered the decay, we've used xylitol and fluoride toothpaste and flossed, and the decay has not gotten any bigger. We also night weaned. I know fluoride is risky/questionable which is why we alternate it with the xylitol toothpaste. That's what works for our family.