It's very common for Cocker Spaniels to get ear infections. They can be due to yeast, bacteria, or both. Most times there is visible discharge in the ear, but sometimes there is very little that can be seen.
I recommend having him checked by your regular vet. I would start with a physical exam and ear swab to be examined under the microscope. Based on this, your vet should be able to tell you if he has an ear infection, and which medication would be the best to treat.
Redness of the ears, head shaking or pawing at the ears. Sometimes all are present, sometimes none!