What type of specialist do you see for back pain caused by pelvic issues?

[StressedOutMom10]

My ten year old son has had dibilitating back pain for seven months.  The X-rays and MRI of his back are not showing anything ‘remarkable’. The doctors have also ran an ultrasound, for any stomach issues, with no findings.  The physical therapists have noted his left hip Is rotated some and is causing a leg length difference.  The chiropractor has noted his pelvis is not supporting his frame. We have found chiropractic and PT do get his hip in alignment however it isn’t staying stable.  He’s in excruciating pain from just walking a city block. Do you have any suggestions on what we should do or specialist we should see?