Without a detailed examination the general consensus of vet, farrier and physio is that his back/hip pain is very possibly linked to his poor foot form.
But until his feet are sorted out we won't know for sure - unless I guess we go for the whole nerve block routine, but we'd have to do that for four legs... as there is only one leg he isn't lame on and that's not much use for walking on.
He has had one corrective trim so far and with 24/7 mooching on the circuit over the past 3 weeks this does seem to have led to some improvement. However it is obviously going to be a long process, possibly as long as a year.
Logic and my purse dictates that I should return him to his owners to sort out. But my heart says otherwise - what would you do?