I see little swollen bumps, especially around the armpits. I also see the red on their chest. These areas are not typical for seasonal allergies, and combined with the bumps, I think you have something else on your hand.
From 10 Signs Your Dog Has Seasonal Allergies: East Shore Vet
If your dog has always been itchy, my guess would be a food allergy. I know that you said that you've changed his diet, but depending on what he is allergic to, those new diet changes may still have had the allergen! For example, I recently discovered that one of my dogs was allergic to poultry. So when I switched from food to food to grain free, the allergy remained because pretty much every commercial brand of dog food and treats had that ingredient.
Also, when you are giving your dog treats, this includes more ingredients into your dog's diet, making the issue of finding the allergen more complicated. It can take months before your dog's immune system calms down after a change in diet to notice the change. So if you are looking for a food allergen, it's best to keep the dog on an extremely, extremely strict diet, so you don't have to repeat the process of weeding out ingredients longer or get bad results (say, your dog had a chicken allergy and you fed it a chicken jerky strip while on a chicken-free food, so the allergy remained).
I also found this webpage with a dog that has a skin rash that seemed to have similar spots to yours .