I don't think you can. Adults make their own decisions, and you just have to let that happen.
Which might mean that there are two meals. Or, you just make the meal, and he either eats it or makes his own.
"never let the truth get in the way of a good story "
my primal journal:
I guess I'm lucky. My husband will eat anything I cook, although we usually cook together. He does the meat, I do the rest. Sloooowly, he's coming over to the primal side. I don't think he'll ever give up his 1/4 cup of cereal in the morning, but he does now eat some meat with it (bacon or leftovers), along with unsweetened berries.
And he'll never give up his popcorn, licorice, and fruit ice-pops (lots of sugar but oh, well). BUT he'll eat nuts for a snack now instead of peanut butter crackers. He eats lunch out a lot, but will get a burger or sandwich and not eat the bread part.
I'm the health nut in the family, and for the past year or so I've just been giving him short summaries of interesting research stuff I've read about; just a few sentences, we might or might not discuss it, and I never try to "force feed" him primal information OR food. If he really doesn't like something I've made (can't recall that's ever happened), he knows he can say "yuck" and get himself something different.