There is great evidence out there that shows that adopting a change of diet, one can reverse tooth decay. I have been blessed to not have to deal with this issue but have a close friend that is presently taking several supplements to assist in tooth decay reversal. Accoring to wellnessmama.com : taking high vitamin butter oil and fermented cod liver oil offer the beneficial omega 3 fats that assist in tooth decay reversal.
I also use the remineralizing homemade toothpaste that is super! It contains no crazy chemicals, is very easy to make, and cost effective!