Allergic Reaction with severe red skin with black marks.

Our boy will be 4 May 4. We are at the vet constantly. He has been on constant meds for two years. He was on metronidazole for yeast dermatitis and Apoquel 2 times a day. He kept getting worse even though we changed his food and shampoos. Now is has gotten black marks on his stomach skin. Now they put him on Cephalexin every 12 hours and prednisone 2 times daily every 12 hours. Plus they gave him a new shot just out. It was $89.00 just for that. He will need another shot in a month. He is allergic to beef and all kinds of grasses. Can anyone help??