It could be his diet.  Try changing foods to something without chicken and corn.  I fed my dog Science Diet for years and her scratching got bad - like you described - at about 7 or 8 years of age.  I switched her over to Nutro Lamb and Rice and her problems cleared up half way through the first bag.  When all of the Chinese gluton poison stuff was going around, I switched her again to another brand.  I forget the name, but it was an all natural, made in the USA food that was vacuum sealed in an foil bag.  Yeah, these foods are a bit more expensive, but when you spread that out over the 40-50 pounds, it is not that much.  Plus, the health and happiness of your dog will be well worth it. 
  
When I started researching ingredients, I was amazed at what was going into some of the foods out there.