I am sorry that I missed your post originally as my guy has this issue. I tried everything and he always wears a fly sheet with a tail protector. No washes or sprays or anything helped until I tried this supplement.
I introduced it when he was at his worst last summer, and honestly I didn't think it would do anything - but it did everything. He stopped scratching, his hair grew back and it was like nothing was wrong. It's basically an antihistamine for horses. The link is just the first place I found it on google, I can buy it for the $15 range (maybe $18 I forget) from my local tack store, so I am sure you can find it cheaper than the link below. AntiHist