Hello zye1. I know it is really daunting to start with, and so much take in. Our dietician told us most 'fussy eaters' have actually got allergies and intolerances, so it's nice to have a reason for things. My dd has just turned 2 and is allergic to nuts, dairy, eggs, soya, wheat, gluten, and kiwis. There are lots of other mums on the allergy boards who also have children with multiple allergies, so you are not alone.
There is another thread on here asking for meal suggestions for multiple allergies too so have a look on there for more ideas.
Remember you can use dairy which is great because cheese, dream, yoghurts etc are very difficult to find replacements for, and are a major source of fat for children under four. Also when it comes to childrens parties you can find treats quite easily that are nut free (both nestle and cadburys do a list online.) And you can fairly easily substitute egg, and it's not a major problem to take it out when you have other protein options. For wheat free I buy doves farm pasta and flours (waitrose or goodness direct.) For bread we buy EnerG bread which is free from everything!
Ordering food online is a good idea. I use goodness direct and allergy Essex. Also food shopping from superamarkets is easier to do online because you need to spend so much time reading labels, especially in the early days. Do keep checking labels though because allergy information does change from time to time on the same products. If you become a member of the anaphylaxis campaign they send you text alerts when allergy information changes or they've made mistakes.
Like already mentioned above, you can give all the protein food groups, which is good because most children with multiple allergies tend to be on the skinny side! So just make sure you have a carb, protein and veg. Although I know right now it must seem impossible to cook for your DS, you should be able to provide nearly all the meals you were already. I remember I was a mess when my dd got diagnosed. But taking out the foods she was allergic to made her into a gear eater and she definitely isn't fussy anymore. You will see a huge improvement in so much by taking these foods out, so you will end up being able to offer more foods than you could before because he will happily eat more foods. And that alone makes it worth it.
So for example this week my dd's meals are spaghetti bolognese (I use rice noodles), chicken and mushroom risotto, roast drumsticks with roast veg, baked beans and toast, lamb curry with rice, tuna pasta bake, jacket potato, homemade fish fingers (you can buy wheat free breadcrumbs.) Orgran is a brand that do lots of free from foods that I use a lot, and they give you quick options when you haven't cooked liked tinned wheat free spaghetti. It's a free world and look what we found are both good ready made meal brands, available from allergyessex and some from waitrose. I do my online food order from ocado because I find a lot more options with waitrose than the other supermarkets. But they all have a free from aisle/section. Sainsburys is also pretty good.
For eating out we go to McDonald's! It's all nut free and dd can have the fries, but that's it! Pig in the kitchen is a good website for recipes. I'm hungry by Tanya wright is a small allergy free cook book that I find quite useful, especially for snacks/party foods.
It will get easier, I promise you. And your DS will be so much healthier for it, so you will see the benefits fairly soon. Anyway I hope this helps.