You know, my son was the hardest to get to brush. When his teeth would start to look orange (yes, that's correct - and gross), he would brush. Just so he wouldn't be embarrassed. Floss? Forget it. He's 18 years old and has had one cavity. One. My brother is/was the same way.
I brush with prescription fluoride toothpaste twice a day and Sensidyne (sp) Pronamel twice a day AND use a flouride rinse. I had 11 cavities at my last cleaning/checkup. I've had 4 root canals and 5 crowns (one for a mysteriously broken tooth - hairline fracture that can't even be seen, only felt when I tried to, you know, chew). As a kid I was on fluoride vitamins.
I've had problems with dry mouth since I was a kid. The medications I'm on now don't help.
While this is a natural consequence for difficult child 1, I wonder if his illness (Crohn's) or any medications for it could be contributing?