go and make a blood test! Maybe he got gluten or histamine intolerance! Histamine is a substance which is pretty much everywhere. Cheese, sauerkraut, fruits, vegetables, especially tomatos (everything which ripens)...
As soon as your son eats sth histamine rich the histmaine level starts to rise and the body reacts with a allergic reaction. A blood test should clear it up. I'm senstive as well and I'm using Daosin, it avoids the allergic reaction and I can eat everything without side effects as well as other problems!!!
BEST OF LUCK!
Have him tested for Gluten intolerance. All too often gluten intolerance is wrongly diagnosed as eczema. That would be my recommendation.
I have personally been down this road myself and suffered for years before it was discovered that I have Celiacs topped with intolerance to night shade veggies & fruit.
Have you seen an allergist or dermatologist about this, I think it would be wise to do so if you haven't.
It appears that his allergies/skin disorder, is getting out of hand, he needs more than OTC preparations at this point.