Re: severe hip pain
How old is your son?
How long did they use a standard cast to keep him immobilized?
How often does your son receive physical therapy?
Is the pain in the pelvic socket itself or in the surrounding muscles?
I am 39 years old now so it has been some years since I had this procedure. However, if your son is young enough (still developing into his adolescence) the speed at which his bones and muscles are trying to develop combined with a lack of movement and regular stretching (caused by the immobilization of casting) could certainly have inhibited his muscles' ability to stretch the full length of his growing bones, thus not making it possible for him to stand upright when the casts were removed. And, of course, causing the subsequent pain.
Because I am a patient and not a doctor, if the above explanation does in fact fit with your son's situation, I would suggest you consider asking your doctors and care providers if some regular hot tub therapy and splint stretching could be used to your son's benefit, before or in conjunction with the more invasive procedure. Also, you may want to consider asking them if they see any indication of twisting of the femur as a result of his recovery issues. This will do two things: it will serve to let them know to what degree you expect to be informed of his condition & their specific answer will give you a jumping off point for what to expect their next treatment recommendations to be.
Best of luck,