Sweet itch is an allergy to the gnat saliva when they bite your horse. Usually over the tail or the mane.
3 years ago I started my horse on flax seed and it greatly dereased his allergic reaction. It also helped decrease the size of his sarcoid on the inside of his thigh!
Diluted listerine on the bite sites also helps relieve itching.
What really helped was turning my horse out at night if at all possible. He loved it! No itchys!
In day turnout he still gets itchy with all the biting though...the only real tihng that could help more is a flysheet. But I think those are WAY too hot even for how light and airy they are. They still sweat.
So: night turnout, diluted listerine, flaxseed.
Vaseline might help too...but be careful if turning out in sun (it could burn him as oils magnify the sun's heat) or if your going to show the next day! It would look lovely with all the gunk stuck to him