Coconut oil or hemp oil
diatomaceous earth, baking soda, bentonite clay, activated charcoal
Real salt from utah

I use all of those for oil pulling - gently - a few times a week. I don't use all at the same time. Most of the stuff is from amazon and the charcoal is from the pet store.

You will be astonished at what comes out of your gums. Brush your teeth and rinse with the salt water first. You start with a really small amount and swish it around. Then spit in the toilet. Then brush after - also gently.

If your son won't tolerate oil pulling - you can try brushing with any of these combos.

This is all over Pinterest. I'm on a natural teeth board with a bunch of ideas. The clay and earth are what remineralizes, I think. There are also a lot of food based ideas of what to eat and not eat for your teeth.

Good luck - a lot of people have good success.