I am sorry to hear that your lovely boy is struggling, it must be very hard to see him in pain. It sounds as though you are trying lots of different approaches for him which is great, although sometimes it can be difficult to get 100% resolution particularly when the conformation of his legs aren’t quite right.
If you haven’t already I would advise chatting further with your vet about the long term options for your dog. Your vet can tailor an exercise regime specifically for him. Physiotherapy and swimming may also be an option for him to help build up muscle mass on his back legs. He may end up requiring medication long term however with careful monitoring and checks with your vet, there is no reason he can’t continue to lead a long happy life with minimal pain.
Hope this helps.