Jeez - I feel for your predicament.
My advice would be to model appropriate eating behavior and keep the lectures to a minimum. Maybe offer to make him something when you're making yourself something. Or offer to cook dinner for the family or maybe pre-make some stuff that he can eat in lieu of the fast food? Kids are all about convenience, and if you can't count on your family to help him make good decisions, and you want to help him, you might have to pick up some slack while trying to teach him along the way to make healthier choices.
For me, the Super Size me movie just about did me in for fast food for good.