D.K.
If he is eating fruits, whole grains, meats and drinking his milk then I wouldn't panic over the lack of veggies. Try putting things where he doesn't even realizing he is eating veggies, even tomato sauce is substantial so spaghetti, pizza are good alternatives. My kids boycotted peas until I stopped cooking them and give them to them slightly thawed out, not only is it more nutritious that way they loved that they crunched, so they are huge fans now of raw veggies and dip.
If you are concerned he isn't getting what he needs try those first Flintstones, they are for kids under 3. That and Pediasure has tons of vitamins and minerals in them and I would sometimes substitute that instead of milk with breakfast. Other things are Carnation instant breakfast with their milk, they think they are drinking chocolate or strawberry milk.