I have 2 yr old twins. Basically, the only thing that works is tough love. YOU have to be wiling to say "I am going to give my son XX for dinner and if he doesn't eat it, there is nothing else until tomorrow". Until you do that, he has no reason to try new foods because he knows you will continue to buckle under and make him mashed potatoes.
We went from having our son eat absolutely NO fruits and vegetables, and only fruit juice (no water) when they were just turning 2 - a nutri grain bar for breakfast, another bar for lunch and breaded chicken for dinner, goldfish throughout the day, and that was it. We'd read about all of the negatives surrounding giving your children processed food, and we switched their eating habits. In one day, we threw out all the bread, cookies, candy and crackers in the house. For supper, I would sit my children's plate in front of them (which was now grilled chicken and veggies, and a little brown rice (flavored with something, of course, like chicken broth, etc). For an entire week he would throw himself on the floor and boycott whatever it was. He refused to try any fruit, and wouldn't eat the chicken because it was naked and had no breading.
It took about a week. Finally, he got hungry and he realized the other stuff wasn't coming back. Now, a month later, he loves naked chicken and "bean-beans", begs for melon and eats an apple without me even cutting it up. He'll drink 3-4 glasses of water a day, and eats raisins as a snack instead of goldfish crackers. There are still things he won't eat, like sweet potatoes, but that's okay, I allow him to eat a few more veggies instead. And they also know that if they don't want to eat their food that's okay, but mommy's not making anything else until the next meal or snack time. They're cool with it.
Think of it like being a test at school. You don't expect your kids to "try" taking tests. You say "tomorrow's the test, that's the way it is. You don't have to get an A, but there will be a test". When you decide he will eat a healthier diet, and that's the way it is, that's when he will.
Note: I focused on only my son, even though his twin underwent the same food "makeover". She never had a problem with switching foods - he was the one who would throw fits, so I gave you the extreme. :)