Soda and Meat.
Clearly you married a male of the species.
The caveman in us all loves meat because its big and drips and tastes awesome. You must accept the notion that he may be permanently vegan-free at this stage of his life.
I'd suggest using his appreciation for meat as a starting point. With his next steak, add sauteed onions, peppers, and garlic.
Chicken has Parmesan written all over it. The sauce is the thing...hard to say no to a rich, thick, flavorful, chunky tomato and vegetable sauce. While you're at it, serve it the chicken parmesan over whole wheat pasta. Roast Beef...in a sub roll...with sliced tomato, green peppers, how about fresh spinach in lieu of lettuce? I'd wait before trying sprouts on the sub...it's hardcore.
Different styles of cooking may be in order. Mexican style dishes are greatly enhanced with vegetables. Tacos crave salsa, especially homemade. Don't leave out the lettuce, tomatoes, onions, cilantro, avacado, and a bit of cheese in there to serve as bait. Quesdillas are served with the vegetables on the side...this would be a good way to introduce the feller to salad. Fajitas have the vegetables on the side, heated...just tell him it makes the meat better.
Stuffed Peppers are sneaky...stuff with meat and rice.
Lasagna...layer in some sliced zuchinni.
Stew.
Hard to beat stew. Meat, gravy, potato, carrots, leeks, mushrooms...I'm starting to drool.
I'd urge you to keep a can of Spam at the ready. Sometimes its hard to wean the carnivores and you will need to prevent shock.