As Nigella says, the symptoms as described do sound somewhat indicative of Perthes disease, and it is possible that this was brought about by the hip dysplasia your son suffered at birth.
It is important to realise that this may well be self correcting over time, as your son grows, but it does need to be monitored, and you do need more information that you apparently have. I would take a notebook and pen when you next see the specialist, and ask specific questions, such as what is the condition that your son currently has, what is the prognosis, what medical and/or surgical treatment is he likely to need, what will the impact be on quality of life, what do they think the most likely causes were, etc.
You then need to review this information with your GP.
I am a little concerned that you seem to want to ascribe fault to someone though. Not every medical condition is as a result of error or negligence... some things just happen.