I feel for you! I have this problem with my horse also. I have exhausted every avenue but the vet and that is my next stop.
He gets sore in exactly the same spot. What made it 90% better was taking him off grain and lucerne hay. He was tying up mildy, which was causing pain in his kidneys, which are located in that area. I swapped his diet around to beet-pulp, copra and meadow hay, and he is so much better. I also got a custom made saddle and a thinline pad... lol. The thinline is amazing, I can recomend giving it a go if you can afford it. The only time he gets sore now is on big weekends away, and I think it might have something to do with being yarded overnight.