Oh, I feel your pain. My son, (who will be 8 in July) was diagnosed with SEVERE food allergies at 9 months. Yeah, 9 months! It took 2 months of throwing up, GI tests, blood tests, weight loss (he was a runt to begin with at 6lbs 4oz)and lots of rashes and tears before we found out he was allergic to everything we were feeding him.
Whole Foods Market was a life saver for us. We switched everything to rice based foods. If you have a Whole Foods or market similar go now! Rice milk, rice cheese, rice butter, rice pasta..........so many things. Hydrogenated soy-bean oil was another trigger and we discovered just how many food products contain this. Label reading is a must, and unfortunately, choices can be pretty limited. For the first 2 years we kept a journal of what he ate every day.
At first, he didn't like the fruits or veggies we introduced, but I think hunger took over. Corn, carrots, potatoes, anything rice based, bananas, apples, grapes, chicken, turkey...........that was pretty much his diet. It was nothing fancy because he was so sensitive, but he slowly began to gain weight.
Does he have a reaction to cooked eggs in a cake? I wound up using applesauce in place of eggs for somethings, and rice flour in place of regular flour. He simply didn't like it. We stuck with the basic foods, I used salt and pepper to add a little flavor, but that was pretty much it. Maybe if you tried simple foods on their own he might like them?
We did a lot of trial and error. The journal helped more than anything. Through his diet, we were able to control most of his allergies (and asthma and exzema). I spent a lot of time online looking up recipes to try, but most of the recipes I tried, he didn't like. They tasted funky, even to me. So I figured if I didn't like them, why would my little guy like it? Even if your son only eats a few things, if he eats them, and keeps them down, then you'll know he's getting some of the nutrients he needs.
To this day my son still drinks rice milk. He's used to it and likes it. He's still allergic to peanuts, egg whites and fish. He's outgrown the rest. Legumes are tricky. He can have peas, but green beans make his mouth itch. He can eat almonds, but not walnuts. This has all been figured out by trial and error. Try a new food item, then wait a couple of days before you introduce a new one. If he is going to have a reaction, it will usually happen in 1-2 days , if not immediately.
Hang in there! It really does get easier.