Head and shoulders works really well! It also works well on fungus so it like a 2 in 1 kind of deal!
Look I have the same problem and I used human head and sholders as nothing else was working and it helped, maybe also just try regular grooming and see if he's sweating under rugs as this might be irritating the skin
I have the same problem with my mare! Especially on the base of her withers and the top of her tail.
My new mare is the exact same!!! My old horse was like that with all his rugs but normally a thorough groom worked for him but not my mare and her two winter coats
It's all over her body especially on her back. I've tried medicated shampoo but it still came back 😬
Is it just in her mane and tail or all over her body? If it's just in her mane and tail MTG works great. If it's all over, maybe try putting conditioner on everywhere so the hair can take back some of the oils that the shampoo strips away.