M.L.
I am so sorry! I am wondering if this rash is perhaps caused by a food sensitivity, such as to gluten. I found this very interesting chart that mentions this, down on box 17:
http://familydoctor.org/online/famdocen/home/tools/sympto...
All I can say, is chart his whole diet, any detergent type changes, exposure to sun, any bug bites (ticks)...