Good evening. I understand your father is suffering from metastatic RCC, but the information you have given is insufficient to be able to understand the cause of his lack of taste and appetite. But do not be troubled, you can start with an appetiser stimulant , that may help somewhat. But for accurate treatment, I would need to check him clinically

Be in close follow up with his.medical oncologist.

This is a known side effect of those tablets. Encourage small quantities of frequent meals and plenty of water. Good oral hygiene by means of brushing teeth and mouthwashes are recommended

Please share his reports