Talk to your vet. There are usually different ingredient diets within a prescription. For example if he is allergic to chicken they may have a beef flavored one. My senior dog cannot have anything but Salmon or he gets to itching like crazy. I order his food off chewy.
My dog perpetually licks his paws. My vet recommended I try adding salmon oil to his meal, but I greatly reduced it by switching him to sensitive skin formula. Guy is allergic to something that we have no idea what it is, but he will lick himself raw