From what you have told me, it appears that your dog is suffering from some type of allergic reaction. This could be due to anything such as dust, pollen, fleas. The constant scratching can cause a secondary pyoderma or skin infection and this will irritate the hair follicles and cause excessive hair loss. The first thing to do is to give your dog Benadryl that can be purchased over the counter at any pharmacy. Give your dog Benadryl at 1 mg per pound of weight given twice a day for 1 week.
If there is no response in a week’s time, you will have to have your vet examine your dog to see if we have missed something such as parasites or other infections. Excessive shedding can be due to stress or a fever as well as scratching. Be sure he is clear of infections or parasites. Mites are unlikely because this at his age will have a specific distribution such as around his eyes and face. This should, however, be ruled out if he has not responded to the Benadryl..