Unlike humans, dogs don't get bored eating the same thing every day. In fact, you can cause health problems for your Cocker Spaniel by changing food too often. It's better to find a quality, additive-free dog food that you can rely on for good nutrition and feed it every day.
When switching to a new food, mix the new into the old, gradually increasing the proportion of new feed over at least ten days.
Adding a few tasty morsels now and then to satisfy your need for variety won't give your Cocker an upset stomach as long as the majority of his diet remains consistent.