I was about to post something similar! My pony has been getting bitten since last year and reacts quite badly to them, he comes in in welts which can swell up and cover an area the size of my palm, then when the swelling goes down or busts through the skin he is left with these small hard lumps. He has some lumps that have been there since October, the scabs just won't go away! My eventual conclusion was ants after I found a flying one attached to him when I clipped him. I hadn't seen any signs til then. Apparently they inject a toxin when they bite that some horses react to. What I'm not sure is how to get the bites/scabs to go away as like your horse they are under the saddle and I worry they will be uncomfortable for him. I don't know why they are biting him and not the others, he is the only black horse and I know some bugs go more for dark horses.