Hmm, well now this sounds a bit different? Typically short haired breeds, don't have a lot of itchy issues? So maybe, it's not "Food?" It's environmental? Does this happen all the time? Or does it happen at certain times a year?
In any case, a Keno-log Shot, would provide "temporary relief." Three to Four month's usually. And usually the "itchy issues," crop up in the Spring and early to late Summer? That's stuff I know from my GSD, I have never had issues, with my Boxer's or other short haired breeds.
When does the issue happen? Time of year?