Odd. I can't say that I have heard anyone complain of an IBS-related rash since I've been on here. Given that you have allergies, it's quite possible that he has inhereited a propensity to have allergies--although his actual allergies and reactions may be quite different from yours. And it doesn't necessarily have to be food group related--I would think it's possible to be allergic to rice but not wheat. After all, I get a very bad reaction from sugar alcohols, which are made from real sugar--which doesn't bother me. All other forms of artifical sweetners don't bother me either. And alcohol, to which it is molecularly similar, doesn't bother me. Of course, I can eat scrambled eggs but not an omlette, so some things just defy explanation.
Have you tried your son on organic foods? Some people cannot tolerate dyes, preservatives, hormones, pesticide residue and the various other non-organic things that are on or in our foods. That may be why his food intolerances seem to be all over the map.