I am SO sorry you and your son are going through this. It is just heartbreaking when something is wrong with a child. I would say much as the rest, check out food allergies and even your clothes detergents and other chemicals around the house. I had a horrible rash on my arms for years and the doctors couldn't figure out what was going on. One day I saw some new detergents on sale and purchased a "clear" no dyes and no perfume detergent to wash clothes. From that time on I didn't have the problem.

I wouldn't give up -- keep checking with various doctors and doing online searches. I have daily searches set up in Google on various medical conditions and I get a daily email with posts matching my searches.