Good.
If he's fine with the other stuff, then keep him on the other stuff. As you say, easy enough.
You know that old saying (though I haven't heard it lately and it likely dates me!)... 29 million (or whatever) Frenchmen can't be wrong... Well they can be. Just because most all the cats in the Universe don't have a problem with a particular food doesn't mean yours doesn't have an allergy to it. It happens. I know this for a fact. Happens in people too. I almost died (and I mean that quite literally) because I was given a drug for arthritis which my specialist assured me is given out like candy, it's so harmless. And it obviously is. But there are always exceptions.
Allergies can also develop, so maybe you 'got away' with it for a while but ultimately (the big bag) ......
Just find what works and keep doing it.
And, once his scabs heal, give him a scritch for him from me and mine.