The vet rang me yesterday.
They sent his x ray to an orthopaedic vet.
He looked more at the whole picture than just the hip and is convinced the issue is with his back or more specifically the discs in his back. He thinks that one of the discs is damaged and pressing on the spine. That explains why his back is hunched and he has some weakness in his hind legs.
He also thinks the degeneration is large and progressive.
It is something you see commonly in dogs but less so in cats.
We are due to speak with my vet on Saturday but I have advised insurance or no insurance I am not putting him through spinal decompression surgery which comes with no guarantees.
I will update after our appt on Saturday and see what we think then.