In addition to your son's diet, you also should watch the products you use on his skin. I have had exzema all of my life and my two daughters have mild cases. When I bathe them I use Bert's Bees products because they are almost all natural. I also use Bert's Bees lotion after their baths. I would try to stay away from cortizone. I have learned that it doesn't heal eczema, but suppresses it. I also read you use Lubridrem, which isn't natural.
I also use Arbonne products, but some people don't believe they are natural products. I don't know, but the baby lotion seems to help. Also, do you have a water softner? That can also help.