Does he like spicy food? I always found chinese food to be a good way to slip more veggies into my diet. Same goes for thai food, indian, mexican... anything with lots of spices and flavorful sauces. 'Course, some of those might be too exotic for him but that's the general approach I would try.
You could also make thanksgiving type dishes like sweet potatoes, mashed potatoes with gravy (there's a simple gravy recipe in la dolce vegan that I like a lot), stuffing... stuff that's familiar to omnis, easily veg*nized, and quite filling. Along those lines, do you know how to make vegetarian pot pies?
Care to give us a list of stuff you already know he likes/doesn't like? Anyway, I agree with Kali about easing him into it. Better to take it gradually than try to make too many changes at once and have him give up.
Catch a shape in the circles of my mind.