my cat has an open sore on his paw that is not healing and I need help please!

My cat has been on Prednisolone for six years due to an allergy where he would itch so much he would get bare spots on his skin.   About two years ago, he developed an open sore on a paw and it has gradually grown larger and now it is bright pink and quite puffed up, but not infected.   I have been trying Derma Gel for one week now and it has not changed.   He also has kidney disease, but no diabetes.   He is around 12 years old.   The vet told me that there is nothing that can be done.   I would like to switch him off the Pred and put him on something else for his skin allergy.   Any of you have something similar to this and have advice for me?   Thanks!

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