You have to learn to let people be. I once had a raw foodist harangue me endlessly about my horrible, unhealthy, environmentally terrible choice to eat cooked food. "All that fuel to cook! All those toxins!" Etc, etc. It was an eye-opening experience and I came away from that determined to never badger anyone else for their food choices. Our dietary preferences are such a personal thing. Have the grace to allow your father his preferences, as the more you push, the more dinner time will become a source of strife. I recommend the cookbook, Vegan Vegetarian Omnivore: Dinner for Everyone at the Table. It includes dishes that are solid main dishes for veg folks that can easily incorporate items that omnis prefer.