I've dealt with this a few times. Taj had skin allergies, and so did Jet, for example.
I have changed detergents to hypoallergenic types, then natural. If nothing changes, I go to the next thing.
When they are allergic to plants, they can be allergic to a lot, year round, but more common is a particular type, and seasonally. Say, spring and summer grass.
Even if they've used it for a while, shampoos that are supposed to be helpful, and that they used with no problem previously, can be a problem. All of mine with skin problems had to have homemade shampoo. Store bought without allergens would have been fine, but they often dried the skin at that point. A simple homemade version can work wonders.
I've also found that a spoon of yogurt per day in their food helps.
Unfortunately, it's very often the grains that are the problem, or a major contributor. So, if he's on a grain food currently, I'd be looking at that almost first.
Some proteins, as well as other ingredients, can cause problems. But, wasn't Charlie doing fairly well on TOTW? If so, then whatever type that was eating would be a pretty safe bet, protein-wise. So, say that he was eating duck. Then duck is probably fine for him.
Just from personal experience, the top problem things for dogs are what I listed:
Their Shampoo/Soap/Other Topical
Seasonal - generally - Plants
Air fresheners/Perfumes/Hair Spray/Certain Oils.
I'd add dish soap for their dishes at this point. And, also rug cleaners. Dogs are on the floor at least part of the time, and rug cleaners are notorious for causing problems in people and pets. I've always wondered what it is in them that makes them worse than some other cleaners, but since they aren't required to tell you what's actually in such products, we may never know.
All you can do is stop one kind of whatever-it-is, and try another. Generally, in a case like this, the most natural alternative that you can think of. For example, baking soda replacing rug cleaner.
Sometimes it's immediately obvious what caused the problem, because removing it from him will give him immediate relief. Sometimes, though, it takes longer, and they continue to scratch for a while. Low dose Benadryl usually helps that, and can give you a clearer picture. It won't mask a real problem, as you have seem.
I'm sorry that you are having to deal with it, and that he is, too. Itching is miserable.
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