Hi C.,
in addition to the other great ideas, I would suggest pesto--which we eat almost daily around here. You can put parsley, spinach, and other greens into it, as well as the traditional basil and season to his taste (probably hold the garlic, or very little!). A great spread on toast, crackers, etc.
Blended vegetable soups that your son helps stir...you can then add the beans and other protein to it to give it texture. Just call the soup broth: e.g. "chicken broth" if it has chunks of chicken in it.
Vegetable pastas. Probably not a lot of veggies, but there are some!
Carrots/parsnips/beets/sweet potatoes/rutabega etc. Try to bring him back to veggies with these sweet roots. You can add honey to them to increase the palatability. Saute until tender. Just call them tubers or something else besides "vegetables."
Quiche or crustless quiche: mix egg/milk with your pesto/greens and a touch of salt and put into a buttered baking dish for 15-20 mins at 350 degrees.
Also, I know that they are not high in some vitamins, but mushrooms like crimini, shiitake, portobello, etc. (not the white button mushrooms, those are not good for the gut) but they are beneficial to health in other ways.
Good luck!