Dachshund Skin Problem?

12 y/o Dachshund has had increased skin dryness and bald spots over the last month. No new shampoos or changes in diet. He did get neutered about 3 months ago after being diagnosed with a testicular tumor. But we didn’t notice any skin changes immediately after, just weight gain. We have tried fish oil with minimal improvement. Any ideas on what is causing this?? I have tried doing research but can’t find anything similar.

12 y/o Dachshund has had increased skin dryness and bald spots over the last month. No new shampoos or changes in diet. He did get neutered about 3 months ago after being diagnosed with a testicular tumor. But we didn’t notice any skin changes immediately after, just weight gain. We have tried fish oil with minimal improvement. Any ideas on what is causing this?? I have tried doing research but can’t find anything similar.