Lots of opinions here, not all to the point.
First, you are treating his ears, so necessary, and may be necessary longer, depending on what his triggers are.
Then, the only reliable way to figure out food ingredient issues is to change all the meats and maybe the grains in his diet, for closer to twelve weeks total. It's nice if he clears up sooner, but it does take that long to be sure. If you change a couple of the ingredients at a time, he'll be old before you get through the entire list!
Blood allergy testing is not good in dogs for food ingredients, it's about the food trial!
Here in the USA, there are issues with some of the 'grain free' diets containing peas or lentils, so look them up before switching to one of those. It's more likely to be one of the meats, actually, rather than a grain, although anything is possible.
Here the blood tests for environmental allergies are pretty good, but you still need that food trial, so most of us do that first.
I've had three dogs that are allergic to food ingredients, and my current two dogs have that problem, besides multiple environmental allergies, by testing. They are doing well on a Rx diet for their food issues, and then some management for their 'allergic to everything in the back yard' problems.
Seeing a veterinary dermatologist might not be a bad idea, if things don't get under control.
Mary