Have you actually does a flexion test and used hoof testers? That is where I would start. I would also start with the other hind leg first. Have someone flex it tightly for 1 1/2 to 2 minutes and then immediately have someone else trot him off after setting it down.
Then, do it with the hind leg in question. If there is a real hind limb unsoundness coming on, it will show up. [Hoof testers will sometimes pinpoint a hoof problem that you would swear was in the upper leg.]
Then your options are many. You can treat it without diagnosing it by using Legend or Adaquan injections or an oral supplement. [The injections are much more effective.]
If this is a valuable performance horse or a show horse, and you actually want to have the problem diagnosed, a good lameness Vet is called for -- not a country GP Vet. A good lameness Vet usually opts for injections to 'block' out the pain, starting at the bottom and working up. THEN, they x-ray the joint or area they pinpoint as the problem.
Once the problem has been diagnosed, the Vet may recommend HA injections, rest, internal or external blisters, or may even recommend shoeing changes, etc. Without a comprehensive work-up and an exact diagnosis, it is like throwing darts over your shoulder and hoping one sticks. JMHO