Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian. I'm sorry to hear about your fellow Jarvis's loss of hair on his ears.
Is he scratching his ears?
Are his ears crusty at all or are they smooth with normal looking skin and just a lack of hair?
Is he a white cat or is it possible he has some Siamese in him?
Ideally your fellow would see his veterinarian to look for evidence of mites, fleas and ringworm, especially if his ears are itchy or scaly looking at all as these are the most common causes of ear hair loss. Sometimes mites can be difficult to find, especially in early infestations so it may be worth using Revolution every 2 weeks for 2 to 3 treatments to see if there is any response.
Other less common reasons for hair loss are related to endocrine diseases such as hyperthyroidism, or Cushings Disease (an overactive adrenal gland) but these are less commonly seen. Testing is done via a complete thyroid profile or just a T-4 for thyroid disease and an ACTH response test for adrenal disease.
If he is a white kitty they can have a hypersensitivity type reaction to UV rays in sunlight and can actually get a sunburn which can traumatize hair follicles leading to red skin, hair loss, crusts, and ulcers on the ears. Treatment is keeping him indoors during high UV ray time (10am to 4 pm) and baby safe sunscreen applied to his ears when he does go out. If he improves that is likely the cause.
Autoimmune skin disease (body attacking itself) is a possibility if his ears are crusty and ulcerated. Diagnosis is via a skin biopsy. Treatment is immunosuppressive drugs such as steroids.
Finally a condition called Feline Pinnal Alopecia is possible. This occurs most commonly in Siamese and Siamese mixes. The hair loss areas are smooth and not itchy. It may be patchy or involve the entire ear. Hair may spontaneously regrow.
Treatment is usually unrewarding but some recommend Melatonin orally at 1/4mg per pound of body weight every 12 hours. That's 2.5mg per 10 pound cat every 12 hours.
Best of luck with your fellow, please let me know if you have any further questions.