This is called overgrooming.
Overgrooming is a very common behavioural problem in cats. Commonly affected areas are the back, flanks, underside and back legs but it can affect anywhere the cat can reach.
The cause is generally anxiety although it may be influenced by anything that makes the cat itchy, in particular skin allergies. The causes of the anxiety can appear quite subtle to us but can be quite blatant to the cat. The most common cause is the presence of other cats in the neighbourhood or even in your own home. There can be other reasons for the anxiety. These might include a new dog next door, builders creating noise in the neighbourhood, a new human (baby) in the household, an owner going away or the owner experiencing their own anxiety problems (which the cat picks up). Because he has done this for so long it’s unlikely to be any recent event of course.
Often allergy is an important part of the problem and fleas feature high as a cause. I know you said that this is controlled by I should discuss it anyway to be sure.You say he is on “flea medicine”. Check him for fleas anyway…sometimes medications fail. Preferably use one of the very effective and safe topical applications (Advantage, Frontline, Revolution). Note that Revolution will also clear up his ear mites. In addition keep the bedding clean and thoroughly vacuum areas that the cat frequents to clear environmental stages of the flea. It only takes one flea to trigger the reaction in an allergic cat.
The allergy can be to other environmental factors and these can be harder to control....dust mites, pollens, etc. If this is the case you’ll find the problem will come and go. Occasionally it is food but you will need to try a hypoallergenic diet to rule this out. These are available from your vet. Worth a try.
Soalthough the overgrooming can be a secondary issue (to the allergy) it will often occur on its own (due to anxiety) and I think that's the most likely reason in your case. The cat's tongue is very rough and can easily damage the skin. If there is irritation cats can sometimes become quite aggressive in their grooming, tearing the fur out, occasionally producing inflamed patches or sores.
So how do we treat the condition? Usually a small dose of cortisone is effective in suppressing the allergy. If you feel he seems itchy that that is worthwhile. You will need to see your vet for this.
Tackling the reason for the anxiety if that is involved can be difficult, often out of our control. There are two products that can be effective for this problem. The first is Feliway. This product is available on the internet, at pet supplies and from your vet. It is a spray that contains cat pheromones. Cats produce these chemicals from specialised chin glands. You will see them rubbing their chin on household objects, effectively marking these as "safe zones". So you can use the Feliway to mark your home as such. This reduces the anxiety. The product is safe and odourless to humans.
The other product is Clomicalm. This is an anti-anxiety medication tailor made for this problem. You would need to see your vet to obtain it. If you are in the USA it is not registered for cats (just dogs) but this is a licensing issue. It is used all around the world in cats and most US vets will prescribe it for cats. It must be used for several weeks to have its effect so I generally use it in combination with Feliway.
I hope I've been of help.
Good luck, Peter