Your description of his behavior and symptoms makes me think of a severe flea infestation and allergy. Have you tried picking up some Advantage or Frontline from your veterinarian and applying it?
Since he is an outdoor cat, you will likely need to apply the flea medications all year round to help control the flea population and keep him from becoming itchy again.
Other parasites, like Cheyletiella mites, are also a possibility.
