I think you are doing fine, and he is doing fine.
Sure, we want our child's intake to be optimum...but per certain phases or likes/dislikes, or moods or what not, it will fluctuate.
He won't starve or go malnourished. If he is gaining weight appropriately, developing normally, and is alert and healthy... then that's good.
Toddlers go through lots of eating phases...their taste-buds are also developing... and "new" sensations come into play. Also, their emotions are STILL developing too, and per his age, their disposition and sense of self too. LOL.
But don't worry. You give him V-8, and add veggies to his soups, and that it good.
Another way is to get Jerry Seinfeld's wife's book... I forget the name of it, but basically what she does is purees cooked vegetables, keeps it frozen in serving sizes, and add it to ANY foods or meals you cook as an amendment. You can add it to soups, baked goods, sauces, spaghetti, anything.
ALSO, one thing that works for me, it that same technique...I puree cooked veggies and add it to my soups and main course I cook. The kids eat it. My kids like soups, so I make this a lot from scratch, and they eat it. To make it fun, I add in macaroni to the soup too or fun shaped pasta. My kids don't like veggies that are in "chunks." So by pureeing it, you can sneak it in.
OR, make smoothies with it....stick in a straw and let him drink it. The same goes for soups... I have greater success with my kids "eating" the soups if I put a straw in it, and then they "drink" the soup more successfully and because it is more "fun" for them, than with a spoon. And you get less spilling and mess this way.
He probably spits out the corn and peas and carrots because he does not like them. So choose other vegetables. Also, my kids like things that have sauces or Ranch dressing on it, to dip in.
Or perhaps he does not like "chunky" foods and veggies... so try cooking it until very soft, then when he eats it, it will "melt" in his mouth more. Raw veggies at this age/stage is hard for them to "chew" and swallow.
Other good choices are: soft cooked/baked/steamed cauliflower or broccoli with cheese melted on it, very soft cooked asparagus, or Avocado (a nice soft ripe one).
Hope this helps,
Susan