I read all the other responses & agree, just want to add a couple things...
My son will only eat broccoli if it is cooked fresh, and soaked in something tasty like cheese sauce, chinese sauce (like broccoli beef gravy) or buttered & salted...the texture has to be just right, cooked almost to mushy, mushier than I like it. He won't touch frozen broccoli! We try to work in more things like eating cold, cooked peas or carrot slices for snacks, since they are sweet, and I usually will offer him a fruit or veggie along with some cereal or cracker...
You can get a vegetable juicer & juice up fresh veggies (consult a juice recipie book if you aren't sure what to mix). It's best to use apples as a base for the juicing, it tastes good & goes with virtually anything...try apple, carrot, celery, and a little cucumber & see if that is palatable. Usually you'll just taste the apple, even tho the juice looks more like carrot, but you'll get all the good, fresh nutrition that you'd get in juice plus, but it's fresh, so that is always better.
We also take a liquid multivitamin that tastes like mango/tropical fruit, made by Natrol...my son didn't like it at first, & he gagged too. Now he asks to take his vitamins, & I can easily get him to take the 2 tsp. recommended dose (once a day). Along with this, I give him Yummi Bears, I expect they are like the juice plus gummy bears...I got both of these at Raley's. The Yummi bears have all sorts of fruits & veggies & are relatively low in processed sugar, even tho they are like "candy"...
I have talked to lots of parents who puree or grate veggies & mix them into sauces (especially tomato sauce, easy to disguise) to get their kids to eat them. Even carrot bread, zucchini bread, & healthy muffins with grated carrots, pureed squash (butternut is really good for this), pumpkin, spaghetti squash, and other cooked veggies can be successful...I especially like the idea of putting raw veggies in the pasta water while pasta cooks, it will get some of those nutrients in, & start acclimating your son to their taste gently.
I also agree don't push it, when my son won't eat veggies (as he does from time to time!), we just add more fruit & limit other foods so we make sure he is getting adequate fresh, fiber full foods.
Good luck! Hope this helps.