My cat Roger will only eat cat food that comes in pouches or IAMS dry food. The problem is he eats the jelly and leaves the meat, so I throw out food every day. He eats a little dry food if there is nothing else, but is quite small and light. He won't eat anything that comes in a tin, except tuna, which he loves. Can I just feed him tuna? Is that a balanced diet? I've tried loads of different brands, but always with the same result. Help!