Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm really sorry to hear about Pumpkin's hairloss and excessive grooming of himself.
The most common reason for these symptoms is flea allergy dermatitis. Flea allergy dermatitis is horrible. If you don't see any fleas you might not have a huge population. And in allergic cats there doesn't need to be many, it only takes one bite a month for an allergic cat to itch and thus groom himself like crazy. If you have other pets who aren't itchy they may not be allergic. Ideally I recommend topical flea products be applied every 3 weeks in an allergic cat when fleas are at their peak (late summer through the fall in areas with a freeze), or with a active problem and then monthly as a prevention. Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II or Frontline Plus, or the new Seresto Flea collar. I don't normally recommend flea collars, but this one really works and lasts for at least 6-8 months. All pets in the house must be treated as if you don't the nonallergic ones serve as a flea reservoir for the allergic one and you'll never solve your problem.
Inhaled allergies (to pollens, dust mites or mold spores) are another possible reason for intensely itchy skin.
To control allergic symptoms you can give a combination of antihistamines and omega 3 fatty acids. Antihistamines you can try include:
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.
2) Chlorpheniramine at a dose of 4mg per 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 her itchiness. 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.
Food allergy is another reason for excess grooming. You might wish to try a true prescription hypoallergenic food for 8 weeks or so. My suggestions are Hills Z/D or Royal Canin Duck and Green peas. The pet store "hypoallergenic" or low allergen foods are nit restrictive enough to work for most cats. He will need to eat only his prescription food, no other treats or foods during his food trial.
If there are other cats in the home that he doesn't get along with or outdoors that upset him perhaps this is stress related. If so a product called Feliway, which is a synthetic version of a calming feline pheromone, may help. You can purchase this from petstores or online.
If he goes outside other parasites like Cheyletiella or sarcoptic or demodectic mange are possible causes as well. If you have other cats these are highly contagious, so I would expect your other kitties to get itchy too eventually.
If he licks enough to create raw spots then the secondary infections are itchy too, and we must treat those to get him to stop itching/grooming.
For now I would start with continued flea control, an antihistamine and omega 3 fatty acids. If he's not significantly better in 2 weeks then it's time for a veterinary examination to look further for problems and/or a hypoallergenic food trial.
Please let me know if you have further questions by replying to this post.