You should be feeding a toddler solids. I don't recommend breast-feeding once they are toddlers, but it is a personal choice. If you are breast-feeding, this will in no way give a toddler the nutrition he needs. He should be getting all his dietary needs from solid foods and only be using the breast milk as a top up. Breast feeding would not cause rotting teeth - bad cleaning habits and high sugar diets would cause this. Teeth should've developed in the womb and come through in the first year of his life. If he doesn't have teeth yet he probably has a problem (not caused by breast feeding) and you need to see a doctor.
Why do you not recommend breastfeeding a toddler? I would think it obvious that by this age the parents will also be giving solid foods and using breast milk as an extra, so why is breastfeeding not recommended? It provides a lot of nutrients and antibodies still that ordinary cows milk does not, the iron in breast milk is much better absorbed than from other food sources and the breast still provides emotional security if needed. Surely as animals - if we still need milk as a toddler (and most doctors recommend a minimum amount still) - then surely nature still provides breast milk as the best and most natural way to meet these needs?