HELP! My pit bull/american bulldog stratches his chest and chews his feet and tail

Help!! I need advise on my dog. His name is Blitz and he is a Pit Bull/American Bulldog mix. He has joined my family at one years old. He is scratching his neck, ears, licking his paws and biting his tail. I have brought him to the vet who put him on prenisone and antibiotics which helped him twice until he finishes the prenisone it starts up over again. Any suggestions. I have changed his food to all natural, as well as his snacks.
I do noit want to keep putting him on a drug that makes him drink so much water and pee sooo often.