That's a huge list to be faced with so suddenly. I stumbled onto this thread purely by accident and I do think it will probably end up moved to the allergy forum, but since I saw my name I figured I'd offer what I can.
The most challenging thing I see is really helping your son deal with an abrupt change. My ds2 is only 35 months; he's never eaten any other way and he can't miss what he's never had. Plus, the few times he's been accidently directly exposed to an allergen, he's gotten so sick that he really is not even curious about other foods right now.
I agree with the pps who said to concentrate on what you can have. It changes your whole perspective and makes it seem much more manageable. For my son, we've identified a few foods we know are safe and he sticks to that very limited diet. Since he's still nursing, we can trial foods first via breastmilk, so my diet is slightly more varied than his (I get more seasoning
I also agree that, if he is allergic to that many legumes, it is better to avoid them all. Same with nuts. It just isn't worth the risk. Not only are the proteins very similar, but there is a huge chance of cross-contamination.
Spelt is an ancient form of wheat and should be avoided by anyone who is allergic to wheat. Barley, like someone said, is very close to wheat and most people with a wheat allergy need to avoid it, also.
We can't do quinoa, but it is the only grain that is a complete protein. Hopefully, your son would eat it.
We do a lot of bison here, but I would bet the proteins would be too similar to beef. Eggs, of course, are a great source of protein and, if you can get hold of raw dairy, it might be worth the effort.
There are some great rice pastas out there and you've got tomatoes--cool!! We've learned to make a pretty decent sauce from red peppers, but it's not quite the same.
I've also found that it helps to just group ds2's related allergies together instead of listing them all--it seems less overwhelming. His allergies are: dairy, soy, egg, corn, peanut, treenut, wheat, bananas and avocados and latex, mustard, sesame, coconut, barley, buckwheat, quinoa, most fruit and veggies, etc...etc...
And the etc...etc...encompasses the little pesky things like cilantro and cinnamon and capers; wheat includes all grains with gluten; most fruit and veggies means that his only safe fruit is blueberries and his only veggies are carrots, sweet potatoes and green beans. But, if I listed every single grain and every single veggie and every single fruit, I'd just give up. I'd sit in the middle of the floor and cry. Squashing it down a little makes it easier to face.
I'll post again if I think of more...hang it there!!