you haven't exactly told us much about your cat or what your vet's diagnosis (if there is one).
When you say he overgrooms himself- does this mean he's now got a bald spot some where? On the belly or the hind legs maybe?
When did this start? How long has he been overgrooming? How old is he and how long have you had him? How bad is the overgrooming. Is he an indoor or an indoor/ out door moggie? Has he had any other medical problems lately?
Has your vet given him the stroid shots?
There are several reasons why a cat may be overgrooming, the most likely one is an allergy- the tricky thing about this would be to find out against what.
Common allergies in cats are: food allergies (against wheat, certain types of meat sources) flea bites, pollen, dust.
Stress as a pychological reason can also lead to excessiv overgrooming- particular in a multi cat house hold.
Cats will sometimes also overgroom if they are under considerable pain but can't get to the source of pain- by washing themselves excessivly they try to ignore this pain by focusing on overgrooming.
Your vet isn't trying to find out what the cause of this overgrooming is? The seroid shot will probably help short term, but what you really should be doing is eliminating the cause.
In most cases an allergy is the reason for over grooming. Have you switched food brands? Any new treats? Think back and try to think of things you've changed lately. A food allergy will lead to constant overgrooming where as a pollen allergy will be periodically, flea allergy only when the cats got fleas and so on.
Start a diary for your cat in which you write down everything you're feeding and when you're cats been overgrooming particularly badly- this way you might be able to see the grater picture.
If it is a food allergy your only chance would be to avoid the trigger. To do this you'd have to find the trigger first:
You could start on an elimination diet. An elimination diet should last for at least 6 weeks, in which the cat is fed with only one meat source. This meat source must be previously unknown to the cat (to be sure you haven't chosen a trigger by chance)- horse would be a good choice. Also it would be best if you started your cat on raw food as this is the only way you'll be able to know exactly what you're feeding your cat. If the overgrooming does not occur anymore you can start to introduce a second meat type, say beef. Again you should wait 6 weeks to find out if this meat source is OK for your cat before you try out what else he will take. Of course this may not be so easy, as your cat won't like such a dull diet over a period of several weeks- and you must be very strickt.
You can also find hypoallergenic food on the market- in this kind of food the protein chains have been broken down and are thus not recognized by the body to be an allergic trigger.
You could also have the vet take some blood and send it in to a specialized lab where the blood would be checked for antibodies against common allergy triggers.
Good luck in finding out what's causing this overgrooming!
p.s. You should maybe consider changing your vet if your vet isn't that interested in finding out the cause and would rather just give him steroids.