Hi, I'm a licensed veterinarian and I'll be happy to help.
You can try one of the following antihistamines:
Benadryl, 25 to 50 mg every 8-12 hours,
Chlorpheniramine Maleate, 4 mg every 12 hours,
Claritin or Zyrtec, 5mg every 12-24 hours.
Also, a daily omega-3 fatty acid supplement such as salmon oil or krill oil can be useful, 1000-1500mg per 10 pounds body weight, every day.
A soap free oatmeal shampoo, used with cool water every 2-3 days, can also be soothing to the skin.