AvatarNathan Richer

I am not an expert at looking xrays so can’t help you there. however, if that is his front view, i would say that his left pelvis is higher than his right.  if you lay him down and pull his legs straight, i bet his legs are of different lengths. give that a look. however, i would first test the assumption that it is fixable and not a structural problem.

I would try resetting his pelvis: https://www.youtube.com/watch?v=INBY84sqrR0
I would reset pelvis before any athletic activity and then do it again afterwards. if possible, i would do it every night before sleeping – pretty easy to do in bed.