You gained two good pieces of info - one, that sals is likely an issue, and second, that you see a skin reaction within 24 hours.
In theory, you should keep everything else the same and take out sals. However, I sooooo remember that desperate for something to eat feeling... What I did once I figured out sals was the issue is added in quite a few foods that were low sals - I kept out dairy/gluten/soy/corn, but I added back in nuts, eggs, lots of veggies, fish, lentils and some other legumes, and a bunch of gluten free grains. That got me to a wider base of things to eat. Then I trialed dairy and gluten, both of which failed for us initially (and I wouldn't trial dairy until you are supping lots of mag - the calcium in dairy runs down mag, and makes sals tolerance worse). Corn still fails for us, and is fairly high sals, and soy uses up the same pathway as sals, so I'd leave those out a while if you can.
The supps that will help with sals tolerance - mag, molybdenum, and B6/P5P (P5P is the active form of b6). Since your guy is little, first, I'd try magnesium via Epsom salts baths (2c in a warm water bath for 20 minutes, every night). Then YOU take P5P, it passes really well through breastmilk. You should also take lots of mag, it can help if you're deficient (normally the level of minerals passed in breastmilk stays pretty steady). For molybdenum, eat lots of beans/legumes or supplement with drops from Allergy Research Group/Nutricology. If you ate lots of beans before, my guess would be it is either mag or b6 that is in low supply. Also, some people need to eat lots of sulfur foods to help with sals (and some can't handle sulfur foods at all - you'll likely find out when you reintroduce eggs!). My guy needs lots of sulfur, so he eats eggs every day.
And yeah, you need to give up grapes - I live in Oregon and we have raspberry bushes in our front yard - trust me, I know how much the no berries/grapes/apples thing stinks! But even now, when his sals tolerance is much higher, fruit is still tricky for DS.
Oh, one other thing - try adding fish back in if you can, and consider supping omega 3s. What we found is that balancing omega 3s and 6s really helped with sals tolerance (there is research to back this up). That really means eating more fish and omega 3s, but more importantly, avoiding omega 6s as best as you can. We eat nuts, but I avoid nut/seed oils (especially sunflower/safflower, they are really really high in omega 6s). Canola is better, low sals and good omega 3:6 balance. And you might trial butter fairly soon as well. Fats are really important on an ED, but tricky if you're trying to avoid both sals and omega 6s.