There is a book called "Food Chaining"
http://www.amazon.com/Food-Chaining-Feeding-Problems-Chil...
The premise is... if your child will eat a french fry - change it up slightly so that it's still a "french fry". Like perhaps Sweet Potato Fry and then start making them less crispy and little mushier and then move into an actual sweet pototo.
In other words... whatever they do eat, move slowly into a more nutriet-dense version of it. And then evenutually it may become a whole new food group. Be creative and be sneaky if you have to.
For instance - start making your own yogurt. Make it sweet at first. You can make it much healthier than yobaby if you research it. Repackage it in yobaby containers if you have to. (Yes, I've done it!)
If you can afford a good blender like a Vitamix or a Blendtec. Make "Ice Cream" or "Ice Pops" with spinach and avacodo or anything else you can connect - but they need to taste good, not like spinach! We eat these several times a week. My kids BEG me for ice-pops. And they get fresh but frozen spinach several times a week.
I also blend veggies like carrots, oranges, apples, spinach etcc... , even cooked chicken into pancakes. My kids like that our pancakes are "green or orange", etc. Again, even eating a pancake was a big step for my kids. Now that they are eating pancakes, French Toast (then they get eggs) was not that big of a leap. But they wouldn't touch it when they were real young.
Will he eat Pasta. change it up with fun shapes and swirls. Do some brown rice pasta if you can for some different nutrients. My kids like olive oil and pink sea salt (minerals) on theirs. I'll change up the oil every now and then to like pumpkin oil, hemp oil, sunflower oil, sesame oil now and then. (Again, different nutrients).
If he likes juice. Make him a smootie. My kids wouldn't touch smoothies. First I got them to like juice boxes. Then we moved into the Odwahla Juice (expensive I know) and then I could finally make them my own smooties at home with a Blendtec and add in some high nutrient stuff like chia seeds, acai powder, greens - but sweeten it, so he is excited about it.
I'm sorry you have to go this route - some kids are just like this (I have 2 of them). And no "explaining" or "reasoning" or "holding out" will do.
Just remember -
1) "Nutrient Dense" for whatever foods he does eat,
2) make gradual changes,
3) be VERY creative and do lots of research. It takes work.
And don't feel bad about investing in some good kitchen tools if you can. Just think of them as an important investment. A professional blender saved my life. and we use it everday. A yogurt maker if you think you'll use it. (I've been wanting myself to get one of those spiral vegetable slicers.)
But I disagree also with your pedi. Your building a little human at this age. Nutrition is so important!!