I only read a few posts but have to agree that your son is sneaking the food because he wants control. He probably feels like something he really likes is being taken away and he still wants it bad enough to steal it. Because he is (or was until he was caught) getting what he wants by stealing it, he is learning a very bad lesson.
I think your need to incorporate treats more in your home - within moderation, of course. At the grocery store let him pick out one or two favorite treats and maybe he gets one if he eats a good lunch or dinner, or after school along with a piece of fruit or something healthy.
I have to agree with the genetics of the teeth, but if it is a concern tell your son that he can have his piece of candy, but he needs to brush and floss his teeth right after.
I'll try to make this quick - there was a study where they gave a group of Kindergartners two bowls of dried fruit - apricots and prunes, I think. The kids could pick whichever they liked best and they picked about both evenly.
The second time they were offered the bowls of fruit, they told the kids that they could NOT have the apricots (put a cover over them), and only the prunes.
The third time the kids were able to go for which ever fruit they wanted and they overwhelmingly went for the aprocts because they were "forbidden" which made them MUCH more desirable.
I give my 3 yo son about a half measuring cup of Sprite every day for lunch. I don't really like doing this, but he loves it and if I say no, no, no then he will want it even more. Moderation is key.