In the UK we call scratches Mud Fever, and because it's so darn muddy there we are all reluctant experts at diagnosing and treating it. It sounds like the problem is so bad now that I can understand that you are not sure which it is, but on the basis that it has been going on for years, and the he SCRATCHES himself, I think that it is Mites.
It sounds like it is so bad that the vet will have to come in I'm afraid.
A friend in the UK had Mites as an ongoing problem with her horse. This was her regime:
- he was bedded at night in straw, but when she first got him she put him on shavings to reduce the 'itchy creature' population. Whatever bedding she used, it got a throrough shake-up each day to prevent populations forming.
- she always kept his legs clipped short and smooth
- she was scrupulous about keeping his legs clean, and would bathe in gentle shampoos, never anything that could irritate already irritated skin.
So, he did have an ongoing problem with mites that were attracted to his big hairy feet, but she kept it at bay.
My other friend at the same yard had a gypsy cob with big hairy feet that she couldn't clip because he was a show champion and needed to be kept natural. He would sometimes get an attack of the itchy mites, so she got the vet in for an injection to supposedly get rid of them, and she kept him on rubber matting with shavings.