I think the vet is your best bet. Your physio is (or should be) a bit tied until the vet does something. I take it the physio didn't use any machinery (PEMF, laser etc) to loosen up the muscles more?
Massage and stretches tend to get overlooked but a physio (or chiro or osteo) shouldn't really be working on a horse without a vet referral. A vet can say whether it's a hock problem, a stifle problem, an SI problem or a spinal problem. The physio may even have an idea but they aren't allowed to diagnose. Once they know the cause of the problem, they can start properly treating then if possible. If not, then they can start working on making the horse as comfortable as possible.
Other than massage and stretches, hacking and walking in hand are a great way to build up muscle, particularly if you have hills. Swimming is another great way, if possible. Work him with his nose down as much as you can in order to stretch along the top of his back (so lungeing, or loose work if he'll drop his head).
Not yet, due soon. The physio said he is ok but needs to build muscle on his back end and relax his muscles, sometimes he just stands there with his hind left leg out but a horse trainer looked at him and said he is tracking up fine but his conformation isn’t great and said the same as the physio. I wondered if there was anything i could get for him or more i can do to help him x