The rash/eczema could be due to liver congestion, due to his poor diet and constipation.
I would give him an enema, have him eat more vegetables, and get him on some high-dose human strain probiotics.
Whatever you do, don't suppress the rash (drive it inwards) with any topical drugs or steroid creams. The rash is the body's way of trying to detoxify/eliminate the "crap" in his liver/gut.