I'm sorry, I know you mean well and you want what's best for him, but I think a grown man should be able to make his own decisions, including what he will and won't eat!
Goodness, if you came to my house, you'd find plenty of junk food, including my worst vice, lots of Diet Coke! I know it's bad for me, but I love the stuff. I'd clobber someone before I'd let them throw it away!
When I go to my parents' house, I take Diet Cokes with me because they don't drink them. If they threw them away, I'd be so insulted that I'd leave immediately! I think the coffee they drink is bad for them, but I don't take that pleasure away from them. Everyone has their weaknesses~
I know you said he's overweight. I am too, but I hope my family still loves me regardless.
Sorry, don't mean to come down hard on you. It's your choice to eat healthy, but you can't police what everyone else puts in their mouth, even if it is a parent.