For the last twelve months or so, I have been using a commercial remineralizing tooth paste which along with some dairy consumption and iodine supplementation reduced my need for fillings by two. Unfortunately, there was nothing the dentist could do for my baby tooth that had no adult coming after it. Having a body that did not absorb nutrients for a prolonged time is not great for teeth (Coeliac). Tea drinking can also help keep teeth strong too.
In one of my more DIY moods, I thought about making my own remineralizing tooth paste but I'm not sure I can be doing with all that. If you can, perhaps these articles may interest you. I'm not sure that they work though or are safe so I am not recommending this. You would have to talk to your dentist/ pharmacist for advice.
This one should be fairly safe if you skipped the bennonite clay. Or if you want to do this in full, talk to a qualified pharmacist you trust:
Remineralizing tooth powder recipe:
A review of home made remineralizing tooth paste, complete with costings and recipe: