If I had a cavity forming, this is exactly what I’d do to stop and reverse it:
Diet
I'd center my diet around grass fed beef, pastured eggs, and raw dairy. I’d limit/eliminate carbs and acidic foods and drinks. No snacking.
Optional: I’d consider taking a vitamin D, K2, and magnesium supplement
Breathing
I’d ensure I'm only nasal breathing and that my mouth is always closed, taping it shut at night/day if needed.
Brushing
I’d brush in the morning and before bed (+floss) with nano-hydroxyapatite toothpaste.