Compulsively licks one spot

I'm told this can be linked to diabetes, however our boy is somehow transient diabetic, meaning he's not diabetic all the time, and isn't currently, nor has he ever been on, insulin. BG normal range.

Yet he has this one spot on he leg he grooms (especially when we touch anywhere along his back!!) Compulsively. Like we'll try to hold his head and if we touch his back at the same time he'll lick at the air furiously until we let his head go. Been like this a few years.

It's starting to bother me as he's loosing some hair there.
Took to the vet, all labs fine...no fleas etc...

But is there like a specific test we should ask for that maybe our shoddy vet didn't know about?
Tips?
Should I be worried?

 

This could be a stress-related condition. You could try some calming chews or a Feliway diffuser for a month to see if that helps. If you get these items from Petsmart, you can return them for a refund with your receipt if they don't work... even if the package is opened and the product has been partly used.

 

Hi! We have used feliway for years, and no stress inducers here.

 

This is just a wild guess, but maybe an allergy?