Getting him on a probiotic and also fish oil/Vit E will help his immune system tremendously. Since we live up north and don't get a lot of sun, I'd also think about giving him some extra Vit D. And I'd buy these supplements ONLY at a natural/organic whole foods store.
I agree with Becky; it's amazing how many problems our body has with allergies and other internal problems that "pop" out on the skin. I'd take him to see an Integration Doctor and have him tested for food allergies (I'd start there first; environmental next, if nothing's found).
My Integration Doctor did the ALCAT food intolerance test on me. It cost $199 and tested for 99 different foods. I discovered I have a ton of food intolerances. Avoiding them has done wonders.