Mine don't eat enough and it's a huge battle. I've tried hiding things in sauces but my youngest knows and then won't eat it. The only vegetable he will eat at a push is carrots, he will eat apples and strawberries but no other fruit.
He won't eat rice, eggs, jacket potatoes, tuna, pork, lamb, duck, brown bread, fish apart from the chippy fish. The list goes on and I dread marking dinners every day because he eats the same crap every day. He also won't touch any multi vitamin!
His brother is similar but he's got better as he's got older. More willing to try new foods, will eat a bit more variety too.
I don't have a clue where it went wrong, I made all of their food as babies, they used to eat everything I made from all kinds of fruit puree to chunky homemade things like risotto. They ate salmon, mushrooms, eggs...now there is no chance.
My husband is as fussy as my youngest and they are very similar. The eldest is more like me and I can see him growing out of his fussiness as he gets older. I eat most things, I won't touch mushrooms, some kinds of seafood and I can't stand olives no matter how hard I try.