Sorry to hear your pup may have HD...I myself have never had this with my three GSDs so I can't help you.
Did you contact the breeder...what was their response ? Is the breeder a member of the GSDL ? Did you see the Hip scores of both...did it say at the top... The German Shepherd Dog Council Of Australia Ink Hip and Elbow Dysplasia Scheme ?
If the specialist says HD...I would take the pup back and demand my money back and go elsewhere because I do know HD is very expensive to fix and no one wants that.