I think that it's definitely time to start talking about how every family makes different choices about food - it won't be long until your DS is headed off to school and making choices that may be different from his classmates in the school lunchroom. Even if everyone brings their lunch, it's really impressive to see the range of what people pack their kids for lunches! Talk up the positive effects of the choices you make, talk in generic terms about why you don't make some of the less healthy choices.
Will he repeat this stuff to his grandparents? Probably. But if he says "did you know, grandpa, that soda can give you cavities?" that's different than "grandpa, mom says you have no teeth left because you drink soda." One is an unemotional fact, the other is hurtful. Just discuss why you do what you do without being judgmental of people who make other choices.
To some degree, I've also had to let go a bit. We have pretty strict rules for the food we buy and prepare in our own house, but we can't (and don't) expect the rest of our families to abide by them in their houses. I'm not saying I would let my child drink a 2-liter of soda just because that's the drink of choice in someone else's house - the girls know their choices for beverages (depending on availability) are water, milk and occasionally juice - but I'm also not going to refuse to let them eat the main course because it has too much salt or butter in it.
I honestly do think that if you talk up why you make the choices you make, your DS will follow along (at least for now!)
Good luck!
Sarah
Mommy to:
Carolyn, 10/04
Anna, 7/08
Matthew, 8/13