My brother has renal cancer stage 4. He is feeling not to bad eating well but his legs are very swollen. Why?

Tumor by vena cava? If your brother's legs are swollen but what is left of his kidneys is adequate, it's likely that some tumor near where the disease began, or scarring of the vena cava which is often invaded by renal cell carcinoma, has interfered with the blood drainage from his legs. If discovered, this might be palliated by his team. Just my thoughts.
Water retention. Your brother's kidney function i presume is not good, so water and urine are retained thus the swelling. I presume he underwent chemo/radiation treatment. Hemodialysis may help. Try these herbal meds to fight off cancer- aloe arborescens and grated frozen lemon fruit. Let's pray for your brother's recovery and strength!