It could be a number of factors - best to focus on one thing and try to narrow it down.
Most skin issues as such are from allergies to food. Does the Wellness you're feeding have grains in it? I would try switching to grain-free (OR, if you were previously feeding a grain inclusive formula, it may take up to 3 months for the body to get rid of it in its system). Sometimes chicken in the kibble can be the culprit as well.
I would add some coconut oil or olive oil to his food to work on the skin in the meantime (even fish oil is great).
Was the Benadryl having any effect at all?