I totally missed your previous update, but I'm glad to hear he's doing well and that the issues seem to be clearing up.
The overgrooming may have been a response to a food allergy, or maybe just a seasonal allergy, like humans get, that was making him itchy and thus overgroom. If it was a food allergy, you'd probably need to eliminate one key ingredient at a time in order to figure out what the trigger was. That can be a long process.
Or maybe it was a one-time thing, one of those mystery kitty ailments that come and go without our ever discovering the cause. Whatever the case, as long as he's back to normal, all is good!