They sound like my son - big eater of "tan" food like waffles, sandwiches, PB, fries and not much else. You have to keep introducing foods but you can do a great job hiding vegetables in their regular food. I made pancakes with all kinds of stuff in them - I used high protein soy and whole wheat flour, flax meal, fruit, even pureed broccoli! I made a whole bunch on the weekends and then froze them with waxed paper in between, then quickly heated a couple for breakfast in the toaster oven. I made "Power French Toast" with whole grain bread with lots of holes in it, and then a power batter of eggs and soy protein powder that soaked into the holes. I cooked the slices in the pan and sprinkled cinnamon on them - it turned white bread into the same color as whole wheat bread as far as my son was concerned, so when I switched to better and better breads, he never noticed. The cinnamon makes it sweet without calories (it's also very good for you) and so he needed less syrup. I gave him real maple syrup instead of the stuff that is all high fructose corn syrup. The bottle is expensive but it lasts because he didn't need much.
There are some great books about sneaking vegetables into foods - like butternut squash puree being the same color as macaroni & cheese, broccoli puree going into spaghetti sauce, and so on. Someone mentioned Jessica Seinfeld's book - I know there was a lot of controversy about whether she plagiarized someone else's book. There is also The Sneaky Chef which is actually geared toward getting men to eat better, but you can adapt it.
When I steamed vegetables, I also saved the nice green water in the bottom of the pan, and used it for anything my son would eat that absorbed all the water - rice and couscous, for example. Don't waste it on pasta where most of the water is thrown away. I saved it up in little freezer containers and just heated it when I had enough.
You could try not making the veggies so mushy - maybe they don't like the texture so much?
There is also a fantastic kids supplement called Now for Kids from Reliv. I just came back from a conference of 5000 people and heard incredible stories of how their kids have done so well on this, reducing colds and flus, getting rid of allergies and eczema, and all kinds of digestive issues. I heard from numerous MDs about how they use this in their practices to get kids the nutrients they need. It's a great value and I'd be happy to give you more info. My friend's child got rid of pneumonia-like symptoms and constant use of nebulizers, and another friend's child no longer has his 50 food allergies. It's a great addition to your situation to help you with your worries about him not getting the nutrition he needs.