Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that Coda has itchy, uncomfortable ears that are red and a little swollen and causing him to shake his head.
If he has scratched enough to draw blood the area may well become infected.
You can clean the area that is raw and bleeding with a little antibacterial soap and warm water on a cotton ball, and then pat dry and apply a little Polysporin. You can also apply a little cortisone ointment (like Cortaid) to the areas that aren't raw, but are red or swollen with a clean cotton swab.
If he continues to scratch then putting on an elizabethan collar (lampshade collar, cone of shame) for a few days while the inflammation quiets can help.
To control allergic inflammation symptoms you give an antihistamine. You can give:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats) at a dose of 1mg to 2mg per pound of body weight every 8 hours. That's a half of 25mg tablet every 8 hours for a cat that is 8 to 15 pounds. Benadryl is very bitter. Some cats will drool excessively or even vomit when it is given. It's not harmful, but not worth making him miserable so try a different antihistamine if that happens.
2) Chlorpheniramine at a dose of 4mg per adult cat twice daily.
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat given orally every 24 hours. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats cannot tolerate decongestants.
Combined with high doses of omega-3 fatty acids antihistamines work to relieve his itchiness and improve skin and ear health. I like 3V caps or Derm Caps as they are reputable, high quality products. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 8 pound cat could take 160mg of EPA per day. Antihistamines and omega-3 fatty acids work synergistically, better together than either one alone. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use as his body adjusts to them.
If the raw area gets very swollen or has a green, foul smelling discharge it is infected and he must see his veterinarian.
Please let me know if you have any further questions.