New cat owner

Hello, I recently inherited a cat from my grandma since she passed. My wife and I are also trying to change his food. We were trying chicken and other protein based wet and dry food. His original foods were entirely seafood but my wife read that an only sea food diet doesn’t have very good health benefits for the cat but he won’t touch the chicken even though we’re trying to do it slowly. What can I do to make his diet change easier? We aren’t completely removing seafood. Just trying to give his wet food variety. What can I do to make this easier of an adjustment?