Sounds like he may have dried soap or something there. Or perhaps that area needs to be curried?
Horses are typically itchy after working hard due to sweat drying.
You could try bathing him or currying him really clean.
But there may be something else going on, like a condition of sorts. Could you take a picture of where he is itching?
If he's itching his tail, listerine will help.
You can use a spray on cleaner prior to currying to get rid of extra dust (I recommend Laser Sheen)
"Strength is the ability to use a muscle without tension"