Try switching to a Lamb and Rice kibble, or Venison and Rice.

As a person with food allergies, I can tell you that there is no vitamin in the world that will have any affect whatsoever. You need to read ingredient labels religiously. Start with a lamb/venison and rice formula, give that a few days. The shorter the ingredient list on the kibble is, the better, because you can "process of elimination" it with him.

Keep in mind, too that if it's skin reactions, you want to be sure it's not some mold or pollen in the environment this time of year that's aggravating him as well. Autumn can be brutal that way.