My cat has dry flaky skin and hair loss behind their ear. What is it?

From the appearance in the picture, it looks as though Jet is missing a bit of hair from his ear in addition to the scaling skin. There are several possible causes for this scaling, including allergies, ringworm, and mange. As a starting point, make sure that Jet is receiving a good quality flea preventive. Advantage is a good OTC option, but your vet will have other prescription flea preventives, like Bravecto or Revolution. I also think it's worth having him examined by your vet, particularly because ringworm and one form of mange (scabies) can be transmitted to people and other pets. It's important to rule out those contagious issues! During a physical exam, your vet may recommend taking samples to examine under the microscope, as well as samples for a fungal culture to rule out ringworm. Then, based on their findings, they'll be able to determine the best course of treatment for Jet. I hope that everything goes well!