Hi Dumplin, and thanks for getting back to me.
Based on your description, it sounds as if it is either a patellar or nerve/neurological problem.
As you have been administering the bute for two weeks and there has been no change or improvement, it is unlikely that the treatment is going to be of any benefit. I would recommend discussing with your vet the possibility of taking an x-ray and/or ultrasound exam of the stifle. It needs to be determined where the exact problem lies. I would want to rule out the possibility of any type of fracture, and if all is well there, to be sure that this is indeed a displaced patella.
One of the treatments for displaced patella is the act of walking backwards. however as it sounds as if your horse is unable to do this, it may be a more severe than normal condition. An ultrasound exam will be able to determine the position of the ligaments that hold the patella in place, and it is not an uncommon procedure to cut what is called the medial patellar ligament in order to remedy the situation. This procedure can be done standing using mild sedation and local anesthesia.
I had mentioned also the possibility of a nerve problem. Can your horse feel sensation on the lower parts of his leg? Blindfold your horse on the side of the affected leg, and apply pressure with a semi-sharp object such as a key in various places on the inside and outside of the leg. If he is unable to feel the sensation, it will need to be determined by your vet which nerve(s) is affected. Depending on the problem sometimes a stronger dose of anti-inflammatories (steroids) can help.
If your vet does not have the equipment/capability to perform the needed testing, consider transporting your horse to a Veterinary Teaching Hospital or larger equine facility. Hopefully there is one located within a reasonable distance for you.
I hope this has helped, and please let me know if you have any further questions.
Thanks,and best of luck!
-Dan C., DVM
Feedback is greatly appreciated!!!