It sounds like your dog has atopy
-- which is an allergy to something in the environment during certain times of the year.. pollen, grasses, etc.
This can be very difficult to control and can cause your dog much stress due to the severe itchiness.
Antihistamines alone are effective in some cases.... but sometimes as it progressively gets worse year by year you need to start treating with a steroid or a combination of steroids and antihistamines.
You can also consider allergy shots if you decide to work with a dermatologist on this. This can be the most effective in some cases and don't require medications other than the shots when all goes well.
You can give Benedryl up to 25 mg per 25 pounds -- that is the mot you can give. If that isn't working , I would recommend a visit to your vet to ask about steroids or a dermatologist.
I know that this is really frustrating... another good thing to do it to bathe about 1once a a week with a oatmeal based shampoo... this will help as well, but wont' fix the problem.
I hope that this info helps... if you have other questions, let me know.