I'm going to go out on a limb here and make a pretty broad assumption about you, and then go from there.
You see, you've always struck me as one of those types of persons where people, when in doubt, do the opposite of whatever it is you're doing. Just to be safe. I think people assume that, since you clearly don't have your shit together (and never have) that whatever it is you're doing in virtually any area of your life must be wrong. I'll bet there are some people who know you who sometimes wipe their ass with their bare hand.
So, if you want to motivate this guy, show absolutely zero interest in nutrition. That's right, cram whatever the fuck it is you want right down your miserable little gullet. Whole sticks of butter, Jolt soda by the gallon, feces, whatever. This will straighten your partner out real fast. The longer you try to instill some positive nutritional information in him, the more likely it is he'll resist it. Forget about bringing others into the equation to shame him into it. Regardless of who is telling him to eat properly, he'll see you doing so and almost correctly assume that if you're doing it, maybe he shouldn't.