Two things: Depending on his physical abilities, you can offer him a "dip" (we've used anything from ketchup to thai sauce) to dunk cooked veggies in--makes it fun to eat and also helps the vegetables taste better to his young taste buds.
If chewing is hard for him, could you try a smoothie? I make one daily for my daughter and myself--berries and banana, yogurt or milk, and then either spinach or broccoli (they don't affect the taste). You can sweeten it to his liking.
Another thing we sometimes did earlier on was to give her vegetables first in the meal, before anything else, because when she was hungry she would eat them! Have you tried yams? Chock full of nutrients, and much sweeter than most other veggies.
I saw that someone suggested V-8. Not a bad idea on occasion, but the juicing process takes out important dietary fiber, and V-8 also has a high sodium content. Also, the earlier he gets in the habit of eating actual vegetables, the more it will stick.
Hope that helps. Good luck!