My multiple allergic DD is now 5yo, and things are fine now, and have been for a long while, she's still allergic to everything though
. I totally know what you mean by feeling overwhelmed, I hated weaning her, the experience was so fraught, and it was not until she was perhaps 16 months old or so that we began to have a good understanding of what she could and couldn't eat (e.g. we'd been withholding fish, as I'm allergic to fish, and she seemed to be allergic to everything, but it turns out, happily, she's not allergic to fish). She didn't gain any weight between the ages of 8 months and approx 20 months, she's still shorter than I think she should be, the whole thing was so draining. She breast fed until she was 3.5 years old as there was pretty much no alternative, it was the only thing sustaining her at times, but I felt so worn out and a little resentful of it. The whole extended breastfeeding debate just makes me feel weary, as I feel I had no choice.
Things did improve after a while though as we just got into the swing of preparing her food in a different way, and we got used to our new existence, it really does have a huge effect on the whole family. We still all eat differently, e.g we rarely have butter in the house, just pure spread, never any nuts, etc. She's allergic to milk, eggs, nuts, at your DS's age she was also allergic to soya, beans lentils and all legumes, mustard, squash and pumpkin, she outgrew these allergies though.
So we're in a much, much better place now than we were in her toddler days, I think the place you're at now was the worst for us, so hopefully the only way is up!