IS not that bad Rosy once you get the hang of it. I think the key is to ensure he has a serving of protein and 2 vegetables/fruit per meal.
Egg is not difficult to avoid, and with Orgran Egg Replacer you can do batter or even custard. Scrambled eggs are out of the question, no replacer can emulate them
To avoid wheat, we use corn/rice/millet/vegetable pasta and eat lots and lots of risottos. If your child is fine with glutten you can find wheat free bread but check for soya flour in it too. Baking your own bread is not difficult, baking it without glutten of any sort is a challenge I still have.
Soya is vile to avoid, it is almost omnipresent, it is likely to be present on any food that has been processed like tinned food, and per cooked meals, frozen things, bread and so on. Good thing if being allergic to soya is that it forces to eat more healthyly... packets and tins become a thing of the past.
We have a box with "emergency" food in the car, it has a small tupper with corn pasta, dried fruit (apples and raisins), rice cakes, small packages of juice and rice milk, etc. If we feel like going to a restaurant we ask for a pasta dish with a safe sauce and ask the cook to use the pasta provided by us.
We have found out that cooking the same thing for all the family forced us to keep DS's food to a higher standard as well as make us learn faster.