I am so sorry your pet is so uncomfortable. Seasonal allergies can be very troublesome. There are several things you can do that will help until you can get to your veterinarian. If there are no underlying medical problems, it is fine to give some Benedryl. An average size Australian Shepherd can easily take 25 mg twice daily. I would also recommend that you bathe your dog in some sort of medicated shampoo ... oatmeal shampoos work quite well for this. Use cool (not cold) water and make sure you dry him completely once the bath is over. Leaving moisture on a dog that has skin problems can quite often make them worse. It is also ok to put a bit of cortisone cream on the worst of the spots. Your veterinarian will most likely prescribe some additional medication to help with the itching.
Dog Veterinarian
20+ yrs experience in vet medicine, special interest in Sighthounds, successful Agility competitor