Low waistline and indentation in Aspeger's Syndrome

My son is 8 and going through the diagnosis for Asperger's syndrome, which is looking very likely. My concern is a physical one.
He has an oddly shaped body. He seems to have a low waistline, below his hip bones instead of above. He sits his underpants and trouser waistbands there and has a marked indentation all the way around. He has an awkward gait but my GP does not know what is wrong. He is now complaining of pain at his hip joint. The consultant paediatrician only seems to want to deal with behavioural issues and wouldn't even examine him saying that this is sometimes an issue with autism. But I am concerned, what is an issue with autism? Does anyone know what this is? I am scared I am missing something and he will have long term issues if this is ignored, and he has a hard, swollen stomach and sides too.