Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
You re very kind to take in Snorkle, I am sure he appreciates not being harassed by a dog he doesn't like.
Hair loss on the back, tailhead area, abdomen and rear legs can be related to allergies (food, fleas, inhaled), stress, or anal gland inflammation. In rare cases it can be related to abdominal pain and internal organ disease.
Over grooming can be related to itchy skin or it can be a self calming behavior.
Grooming releases a calming neurotransmitter called serotonin, which is a calming neurotransmitter.
I would recommend continuing use of a flea prevention. I know that you couldn't find any but in allergic cats it only takes one bite a month for them to be very itchy and when they are grooming so heavily they may actually eat any evidence of flea being present. Some preventatives are more effective than others and if he is experiencing an occasional bite that can lead to him overgrooming himself. If you have other cats and they aren't affected than it may be the other cats aren't allergic. I like Frontline Plus or Advantage for cats. Many over the counter products aren't effective and some are toxic.
If he has an anal gland infection treating that should help. If it's a chronic problem then removing them may be indicated.
He may be allergic to a protein in his food. I recommend trying a true hypoallergenic
diet such as Hills D/d or Royal Canin Duck and Green Peas. If you choose to use a hypoallergenic diet then it must be the only thing that he gets for at least 10 to 16 weeks to see the full effects. No table foods or treats or flavored medication. Most people report some improvement within 8 weeks.
If this is stress related then decreasing stress by using Feliway may help.
Feliway is a synthetic analog of a facial pheromone which cats use to mark their home territory and is calming for them. Even though things may seem fine there may things that you aren't aware of. Sometimes anti-anxiety drugs like buspirone or fluoxetine prescribed by his veterinarian may be indicated.
If you think this may be inhaled or flea allergy related then therapy to prevent the allergic reaction with Chlorpheniramine at 4mg per cat once to twice daily and an omega-3 fatty acid such as Derm Caps should help him be more comfortable.
So in short you can have him checked by a veterinarian to make sure there's
no physical reason for his discomfort (full anal glands or abdominal pain). Or you can try flea preventatives, an antihistamine (chlorpheniramine) and an omega-3 fatty acid as well as a hypoallergenic diet and the feliway first and take him to be examined if those don't help. Because he goes outdoors other parasites like sarcoptic mites, demodectic mites, or cheyletiella mites are possible as well and should be checked for if he isn't improving.
Please let me know if you have any further questions.