Are you supplementing any oils?
I would add 1 tbsp flax oil p/day to the vegi mix.
If you find that there is irritation, then you might want to test the protein source you are feeding for allergies. If you are feeding mostly chicken then take a break from the chicken and try another meat source. He might be allergic to one of the protein sources. If this is the case you need to figure out which one.
Are you bathing him with a specific shampoo?
Are there pestisides on the grass he walks on?
Consider where he walks, maybe he is getting irritated from an outside source.