K.A.
Cavities are partly genetic. I have 3 kids. 2 of them have had quite a number of cavities. My youngest had cavities in 2 molars that had only broken through 2-3 months before! They came in defective right from the get go. One of my kids has only had 3 or 4 cavities his whole life and he's nearly 10 now. Same diet, same toothpaste etc. The 2 with a ton of cavities got their father's teeth. My husband had the same exact problem when he was a kid. Took everything just to keep the teeth in long enough to fall out on their own. Dentist confirmed it wasn't because we weren't taking care of their teeth or feeding them wrong, it was genetics.
He did say that using xylitol might be worth a try. He did warn that it can cause a bit of upset tummies though.