This is sort of a new approach, but my buckskin filly did that when I was teaching her to pick up her feet, she would put all her weight on her back end and kinda hold her leg up awkwardly because she wasn't sure she liked me down there.
I was told you can press pretty firm from the withers down to the point of hip in various areas on the horses back to check for tenderness. Don't know if it will work too well for something deeper though.
If he is getting better it could be that he is getting used to you. Jmo though, lol. Keep us updated.