I'm wondering whether he's regularly getting a flea/fleas which bite him? (OK, the flea(s) then die but he's still been bitten.) Do you treat the house with something like Indorex? And does he hang out with the gang in any particular place (eg a shed) where there might be eg a reservoir of eggs?
Your vet sounds a little cavalier if he's got such a problem. Have any tests been done? It might not always be a flea allergy.
My first boycat used to overgroom due to flea allergy, he would lick himself raw, usually on his chest or under a front leg. The vet gave him steroid injections which worked almost immediately, although he could only have 1 or 2 a year because of the risk of side effects. He did get treated for fleas regularly but seemed extremely sensitive to them.