Sorry your DS is suffering, you too for that matter :(.....
just a few suggestions that might be worth you looking into further
"urticaria" & food additive sensitivities, especially "azo dyes" & Benzoate Preservatives"(my own DDs skin problems where down to this)
Dairy, or lactose intolerance, known to cause itchy bums & stool problems, along with other things such as skin problems, my own DD is intolerant to lactose, so is fine with milk, but only "lactolite" milk, normal milk affects her stomach badly, & during a flare up she can't stomach any dairy at all
DD was fine when younger, but gradually thins got worse after a couple of course of antibiotics :(
also look into soy & egg intolerance (they pop up in all kinds of things & aren't clearly labeled unless you know what to look for name wise, ketchup being 1)
Maybe look at keeping a food diary, & maybe try an exclusion diet & watch out for flare ups when he's given food again
if none of that works, then try looking into "histamine rich foods" in some cases it can be a combination of things that cause the "histamine" or allergic response
ANTI histamines work by dampening down the bodies natural defenses to histamine, but histamine occurs naturally in a lot of foods, but you will most likely find its only 1 or 2 of those foods that cause you problems, as they over load your system with hystamine, at a time when your body is releasing it to combat pollen or other allergens.... for example, I can't eat tomatoes in high pollen months with out a hayfever like reaction, fine eating them at other times, ....friends Son is the same with Bananas, but affect his ashma....hope that makes sense 
you'll find lots of info on all the above if you google :)
hope thats of some help & that you find the cause